Because I've never heard this collocation without a definite article. For instance: "You look just like one of the bad guys" or "Damn you, I thought you were the good guys!" And in both given examples I can't spot any reference to some specific guys. Also pleaseee be kind to list other nouns which, like that one, can't get away with a definite article:)
The good guys and the bad guys are standard narrative roles, like the hero and the villain.
Every spectator or reader of a narrative expects these roles to be played by some character or characters, so they are in that sense fully specified within any discourse about the narrative, and entitled to the definite article.
This doesn't mean that the spectator/reader knows which characters in the narrative play those roles (figuring that out is part of the fun), and it doesn't mean that those roles actually will be played in every narrative, merely that the roles are part of the expectations a spectator/reader brings to the genre.
It is also possible to use good guy, bad guy, hero, villain with the indefinite article. If you say "I think Jim is a good guy", or "a hero", you mean something a little different: you think that Jim exhibits qualities which fit him to play that narrative role, without asserting that he actually does play the role in some narrative.