There have been a couple that tried that, Shadowrun and Unreal Tournament 3 spring to mind. Shadowrun probably did better in that it tried to even out both control methods, but the way in which it did that was just by making the auto-lock so heavy that it made the accuracy of a mouse irrelevant and not all that much fun to play as a result.
Was an alright game but not much else. Once the 360 vs. PC novelty wore off it was quickly and deservedly forgotten about.
Unreal Tournament meanwhile didn't allow 360 vs. PC but did support mouse & keyboard on the PS3 with the result that those controls were pretty much banned from every server for being unfair to those playing with gamepad. Go figure.
To be fair there are some genre that are just plain better with a controller as well: Racing games (Though ideally you'd want a wheel), fighters ala Street Fighter/Tekken etc, and good ol' fashioned platforming adventure. Trying to play any of those genre with keyboard and mouse is a pain in the ass more often than not, which is why I own a pair of gamepad for my PC myself.
In terms of shooters however yeah, mouse/keyboard is just better. Same with strategy games, which is kinda a moot point since there are barely any decent strategy games on consoles to speak of but whatever.