If there are so many Jedi that there's even a Jedi Order, how hard would it be to hunt down two Sith lords? Once they found those two, wouldn't that be the end of the Sith?
I'm not versed with the Star Wars canon outside of the movies, so I'll answer using that as a basis.
Firstly, the Sith had the power to conceal themselves from the Jedi. No one knew about Darth Sidious and he was constantly hanging round with Jedi.
Also, the Sith knew that they were more vulnerable than the Jedi (as you said there were only ever 2). This put them in a more precarious position strategically.
This is why they only ever revealed themselves when they were almost guaranteed to be victorious.
In Episode 1, Darth Maul only attacks, at the most, 2 Jedi at a time (Qui-Gon and Obi-Wan), and he almost defeated them both on his own.
In Episode 2, Count Dooku only reveals himself to Obi-Wan when he has imprisoned, and only shows himself in public when he's surrounded by an army of droids and Geonosians. Sure he was almost killed by Obi-Wan, Anakin and Yoda, but as soon as the Jedi showed up on Geonosis with the Clone Army, he tried to flee.
In Episode 3 Dooku is, again, surrounded by an army. But he happens to be killed by Anakin. The only time Darth Sidious is in danger is when Mace Windu shows up with 3 other Jedi unexpectedly. If his plan had worked then Yoda and Obi-Wan would never have survived, so he only revealed himself voluntarily when he had the upper hand.
Episodes 4-6, Vader and Sidious are pretty much the only people left in the galaxy who can use The Force, so there is no one left who is a threat to them, particularly as they're constantly surrounded by their own personal army of storm troopers.
Notice that when Dooku and Maul died, it was because Darth Sidious had pretty much planned it. Which leads me to the overall answer to your question.
Because Darth Sidious is far too clever.
He plays the senate, the Jedi, the Viceroys and even his own apprentices like puppets throughout the entire 6 movies, in order to become Emperor of the Galaxy, all whilst hiding in plain sight.
The only reason he gets killed in Episode 6 is because his own apprentice betrays him. Notice that before that point, he was in almost no danger. If he had not been so intent on gaining a more powerful apprentice and had just killed Luke, then the Empire almost certainly would have won.
Three primary reasons:
First: Until the events of The Phantom Menace, everyone assumes the Sith don't even exist anymore. It's not until Attack of the Clones that everyone is finally convinced.
QUI-GON : ...my only conclusion can be that it was a Sith Lord.
MACE WINDU : A Sith Lord?!?
KI-ADI : Impossible! The Sith have been extinct for a millenium.
YODA : The very Republic is threatened, if involved the Sith are.
MACE WINDU : I do not believe they could have returned without us knowing.
YODA : Hard to see, the dark side is. Discover who this assassin is, we must.
Second: The Jedi's ability to use The Force has diminished.
Windu: I think it is time we informed the Senate that our ability to use the Force has diminished.
Yoda: Only the Dark Lords of the Sith know of our weakness. If informed, the Senate is, multiply, our adversaries will.
(src: Star Wars Episode II: Attack of the Clones)
Third: The Sith are concealing themselves via The Force.
Sidious is a practitioner of Force Stealth, which suppresses his Force-Footprint, if you will. The first clue regarding the Sith in hiding comes from Count Dooku.
Dooku: The truth. What if I told you that the Republic is now under the control of the Dark Lords of the Sith?
Kenobi: No, that's not possible! The Jedi would be aware it!
Dooku: The Dark Side of the Force has clouded their vision, my friend. Hundreds of senators are now under the influence of a Sith Lord called Darth Sidious.
But could Obi-Wan really trust Dooku? Even if you could, they still have an immense number of people to investigate - and would be doing so with diminished powers.
In the end, defeating them is one thing, finding them is the real issue.
The Jedi just believe the Sith don't exist anymore. The Sith have planned to rule the galaxy for centuries.
If you refer to Legends, a thousand years before the Battle of Yavin, the Sith and Jedi were at war. According to Darth Bane, the Sith have become weak, spending all their time fighting themselves. Therefore he destroyed all Sith lords but his apprentice with a Thought Bomb, starting the Rule of Two era (only a master and a apprentice).
The two remaining Sith should hide themselves from the galaxy until they raise enough power to beat the Jedi Order and rule the galaxy. They have this complex plan that will last from Bane to Palpatine. The master teaches his power to his apprentice, until the apprentice becomes stronger enough to kill him.
During Episodes I-II-III, the Sith have become so powerful that, as Yoda said, the dark side clouds everything. They didn't realize that Palpatine is a Sith lord until it's too late and he's actually able to kill almost all of them.
This is my way of seeing things. The Force always balances itself. The Jedi, being many have their force powered distributed among them all. This makes each of them weaker in the force. The Sith however, only have their Dark Force power distributed among them both. That way there will always be one most powerful and his apprentice and only after the apprentice becomes more powerful than the master and kills him, the cycle restarts. This is one of the reasons Bane established the Rule of Two.
Please allow me to offer a quote that may help in reasoning why it could be so hard:
“If I determine the enemy's disposition of forces while I have no perceptible form, I can concentrate my forces while the enemy is fragmented. The pinnacle of military deployment approaches the formless: if it is formless, then even the deepest spy cannot discern it nor the wise make plans against it.”
There is also an important point yet to be raised - the definition of what a Sith is is... vague. We do have examples in the form of Asajj Ventress of Clone Wars and The Inquisitor of SW: Rebels that being trained by the Sith, being a dark side Force user and having a red lightsaber is not enough.
While true Sith are thought to be extinct and two of them there are they could, in principle, train whole armies of not-Sith to challenge the Jedi order.
In the novelization of "Revenge of the Sith" Yoda realizes something while he is fighting Darth Sidious. The Jedi have been stagnate during the millennium in which the Sith were thought to be extinct - they are pretty much the same as they were before. However the Sith have been learning and adapting, preparing to destroy the Jedi. Had the Jedi discovered the Sith quickly, they could have destroyed the Sith with little problem. A thousand years was enough time to plan for only one or two Sith to enact the destruction of the Jedi.
It's like why an amateur would never be able to beat a chess master at chess - the chess master is planning many moves ahead and will maneuver the amateur into an impossible situation well before the amateur is even vaguely aware of the danger.
If there are only 2 Sith...
In addition to the "Rule of Two" Sith, there are other groups of Sith stranded or sequestered on lost/hidden worlds that pop up from time to time. There are Dark Jedi (good guys gone bad) who are practically indistinguishable from Sith (and occasionally become true Sith). There are also ancient ghosts, for lack of a better term, of dead Sith Lords that sometimes possess or instruct Force users and thus recreate the Sith. So in a nutshell, there are not only 2 Sith in the galaxy, but that is not common knowledge in the Star Wars universe.
...Why aren't they easily defeated?
Classically, good guys follow the rules, even in a fight, while bad guys will cheat. Sith don't fight to uphold any ideals, they fight to win, and will do anything necessary to do so. Fighting an enemy like that without stooping to their level is difficult, and incidentally, is part of the reason many Jedi fall to the dark side.
Some fun titles to check out-
- Legacy of the Force series (9 books, starts with Betrayal by Aaron Allston)
- watch several Jedi (including a Solo and a Skywalker) drawn to the dark side.
- Darth Bane series (3 books, starts with Path of Destruction by Drew Karpyshyn)
- when and why the Rule of 2 is born. Also, answers your question...
- or just start with book 1 in any of the Star Wars series from this link
I recommend starting with the "Tales from ________" short story collections listed about a third of the way down the page if you haven't read any Star Wars before. They are remarkable stories, well written, easy to digest, tie into the movies, and whet your appetite to dive into the novels.
Well there are a couple reasons the Sith do the "Rule of Two." One is simply keeping the tradition started by Darth Bane. However, HE and the rest, thought the rule was a good idea because
- it consolidates power to 2 people held above the rest, even if they are dark force users (considered dark Jedi instead of Sith lords)
- it is actually harder for a smaller force to be FOUND if not engaged. They can hide much better BECAUSE they are only two.
It would be much more difficult to hide an entire dark side order and/or evil government and so forth. The Sith deal in deception and shadow, and would not honestly reveal who they are, what their intentions, are, or risk losing in open battle. They are not the good guys and the rest of society with a different color, they have completely different philosophies on organization and gaining power. The Sith seek to take it from the Jedi, who they view are unjustly in power. To the Sith, they are the minority taking the false system down from the inside, and when they are on top, since there are only two of them, they are like gods and don't have to share with an entire order of others.
They not only want to take the Jedi out of the way, but I think most importantly, the rule of two ensures that power won't stagnate: if the apprentice becomes more powerful than the master, there is no use for the master anymore, so each generation of Sith lords in theory would be more powerful than the last. To Sith, everything is about individual strength.
Well you forget that until they reveal themselves in The Phantom Menace the Sith were thought to be extinct by the Jedi that was the whole point of the rule of two to hide from the Jedi and ingratiate themselves into the upper echelons of society and destroy the Republic from the inside and by default the Jedi which as we know Palpatine succeeded at.
There once were many Sith, even a Sith order, but they were always at war with each other fighting for power. The Sith being weakened by these fights allowed the Jedi to come in and kill themk. Darth Bane saw what was happening to the Sith order and being basically the lone survivor, he created the rule of two and went into the shadows to plan and scheme.
The Jedi assuming that the all the Sith were dead went into a time of peace, even retiring their war armor. 1000 years passed and still no sign of the Sith. So my point is how would one find two people in the known Star Wars universe or the outer systems when they don't know about them and the people they are seeking are all time hide and seek champions?
Palpatine's plan to rule the Senate and then the galaxy was Darth Bane's true goal. The Sith learned patience; more of a Jedi trait.