Orcs can be slayed with very little effort even by the un-warrior humans, despite their fully developed figures, average-to-tall heights, muscular bodies. (See The Hobbit series, where a dwarf would dive into a herd of orcs and come out un-wounded and killing most of them). What could be the technical explanation?
I must admit that it's true in the movies they look like having a hopeless advantage. BUT...
Most of the battle orcs depicted in the movie seem to be Uruk-hai, the bigger breed of orcs from Mordor or Orthanc. But in the book, they were not, only the usual smaller breed of orcs, like we actually see in the second wave in the movie, the one conducted by Bolg. Men and elves are much bigger and stronger than those orc races. Also, orcs don't have good equipment like we see in the movie... they are not disciplined...
They don't have all those big monster trolls, and those pterodactyl like big war birds, which are in fact much smaller bats in the book, even though they are big for bats and numerous enough to be like clouds which bring terror to the battlefield, they don't actually have the kind of size or raw power shown in the movie. (Thanks to Keen for raising the point about the bats, which I had forgotten.)
They don't have those earth eaters worms (probably inspired by Dune) or I don't know what Peter Jackson calls them in English.
In the book, the men from Lake Town in the battle are trained warriors, not civilians. This is a part of Middle-earth called "The Wild", and it's a very dangerous place, not a peaceful setting. Men living there are used to battle.
As depicted in the movie, dwarves in Tolkien's world are redoubtable warriors, with much steadfastness... they are really strong and can endure a lot. They are disciplined and they are masters of iron work, so they have really good equipment.
In the book, the elves, dwarves and men know the orcs are coming and have time to prepare, Gandalf, Thranduil, Bard and Dáin devise a plan of attack, they array their troops on the heights north and south of the valley and the orcs don't enter the battlefield by tunnels, but in a frontal attack by the mouth of the valley without much strategy as is their custom, and are attacked on both sides, first by the elves, then by the dwarves and men on the other side. The orcs are massacred right there and would be crushed if it wasn't for the second wave that then came attacking the spurs from the top of the mountain after a climb of the other side.
But what would then maybe have been a lost battle for the dwarves, men, and elves is saved by the arrival of the eagles and Beorn (the man shape shifter who is also a big bear). Beorn gets to kill Bolg, the orc supreme general, and the battle is won there.
So, yes, there is much "Peter Jackson" in this battle, Hollywood style - Tolkien was way more realistic.
Though this may have been more exaggerated in the movies, this is the case in the LOTR books as well. Recall Gimli's and and Legolas's contest at Helm's Deep which Gimli won 42-41 (or 43-42 in the movie).
Possible reasons for the individual inferiority (perceived or actual) of orcs:
Orcs are ugly, half-crippled, corrupted spawn of Morgoth's making. It's not surprising if they are less skilled than a elf, man, or dwarf. Eru/Aulë did better than Morgoth.
Throughout the LOTR/Hobbit movies and books, Sauron relies on surprise and overwhelming numbers in battles, not skill or finesse. He and his generals have little regard for the value of an orcish life, and they frequently strategize for Pyrrhic victories.
Orcs are greedy, selfish, disloyal, and lazy. You can expect them to fight more like rabble and less like a well-organized army.
In the LOTR, orcs attack fortified positions. Man-to-man (or man-to-orc), that is a significant disadvantage.
Our heros, with the exception of perhaps the hobbits, are not ordinary warriors. Gandalf is a Maiar and one of only 5 wizards. Legolas is an immortal elf and son of King Thranduil. Thorin is the heir to line of Thror. Bard is a descendent of the Lord of Dale. Boromir is the son of the steward of Gondor. And so on. In the Tolkien-verse, royalty such as these are significantly more powerful (in battle and otherwise) than a random peasant.
There is much evidence that the original orcs would be closer to what we usually call goblins. Tolkien did not differentiate between the two words and treated them as synonymous; the official explanation being (in the preface to The Hobbit) that 'orc' was the hobbit word and 'goblin' was the human one.
Perhaps that helps you to imagine the original race not as man sized hulking beings like those who have become Uruk-Hai, but as a somewhat more chaotic and physically weaker lot.
Also bear in mind even the larger Uruk-hai orcs were not (in the book) particularly large relative to men. They are described as being 'almost man-sized' when their great size was spoken of, which means normal orcs were considerably smaller than humans and possibly dwarves also.
The movies tend to follow the hollywood formula of one hero or a few heroes overcoming masses of bad guys. In the book the battle scenes were not so simplistic. The only time I can recall the characters taking major risks was when they were attacked in Moria in the Chamber of Mazarbul, where they hoped a quick, spirited attack may surprise the orcs temporarily and thereby buy them some time to escape, and when Aragorn led his army to Mordor to initiate an attack. When they were attacked on the banks of the Anduin, they had no option but to defend themselves, and some survived because the orcs hot-footed away with their prizes (Merry and Pippin).
Orcs may not have been the most disciplined, but they had numbers and a strong miltary culture. There were formidable enemies and your question is more a reflection on the unbelievably ridiculous action scenes we have to endure in all hollywood movies.