Probably because there is just so much less in a fighting game than any non-fighting game.
Now, before all the fighting game fanatics start sending me hate mail, here is what I mean:
Fighting games are all about balance between characters (or the lack thereof, lulz), and the combat. Yes there is a little bit of storyline thrown in occasionally, and each character has their backstory details and whatnot, but in the end, every person that plays a fighting game cares about the action of the game; the fighting.
RPGs like Fallout have character builds, level design, story, etc.
Strategy games have a ridiculous amount of balancing, story, etc.
FPS' have balancing, combat, usually aesthetics, the dreaded realism, etc.
So you see, the reason Fighting games are able to acquire a much higher rating than you would like, is because they are rated on less than any other genre. No no no, I am not saying Fighting games are simple and childish, but a developer can focus all their effort on fewer aspects of a game than say, an RPG, in order to make a better game.
At least, thats what I think.
I personally suck at any fighting game that isn't Soul Calibur 2 or Super Smash Bros.