Since the Triwizard Tournament is the biggest competition in the wizard world, why would it be so easy for someone to bewitch it?
It wasn’t easy to fool the Goblet of Fire.
It’s not particularly easy to fool the Goblet of Fire. Barty Crouch Jr. as Moody said that it would take an exceptionally strong Confundus Charm to make the Goblet forget there are only three schools competing in the Triwizard Tournament.
“Because they hoodwinked a very powerful magical object!’ said Moody. ‘It would have needed an exceptionally strong Confundus Charm to bamboozle that Goblet into forgetting that only three schools compete in the Tournament … I’m guessing they submitted Potter’s name under a fourth school, to make sure he was the only one in his category …”
- Harry Potter and the Goblet of Fire, Chapter 17 (The Four Champions)
He’d know, since Barty Crouch Jr. was the one to fool the Goblet, and he did actually do it that way.
“Who put your name in the Goblet of Fire, under the name of a different school? I did.”
- Harry Potter and the Goblet of Fire, Chapter 35 (Veritaserum)
Therefore, it’s not actually that easy to fool the Goblet of Fire, though it is possible to do it.
While the Triwizard Cup itself is not mentioned as having have any particular magical protection, Barty Crouch had to go to extreme lengths to get access to it, spending 8 months impersonating a trusted and distinguished auror who would not be suspected of interference.
As for simply summoning the cup, it's mentioned that spells and enchantments were placed on the maze as part of the task. It seems likely that one of those spells prevented the competitors from bypassing the maze entirely by summoning the cup.
"There will be obstacles," said Bagman happily, bouncing on the balls of his feet. "Hagrid is providing a number of creatures . . . then there will be spells that must be broken ... all that sort of thing, you know.
Remember one of the opening line of prison break (rephrased):
It is not impossible if you designed the place.
No matter how secure your system is in regard of exterior attacks. If you are the one designing the system, or protecting it, it will be very easy for you to attack it.
All the time some cheating did occurs, it was because either Hagrid or directly MadEye/BartyJr. (Or even Dumbledore if you count the negotiation with the merpeople at the end of task2).
So when the one in charge of enforcing the rules are the one cheating...
Quis custodiet ipsos custodes?
The Goblet of Fire is a magical object. However, everything has its flaws.
In my opinion, the Goblet did not have much to any protection of its own but relied on the safety precautions set up by the staff and headmaster of Hogwarts. Of course, these precautions were for students.
All in all, the Goblet alone is easily tricked but the precautions around the Goblet usually keep it from being. However these precautions (such as an age line and just being in the Hogwarts castle) are more modeled to keep out younger students and people who are not staff or students at Hogwarts.
This is how Barty Crouch Jr was able to trick the Goblet easily as he was much older than the age line required and under the disguise of Hogwarts teacher.