If they allowed modding, people wouldn't get banned and have buy the game again to play.
Of course they've made the game as good as possible. It's actually quite impressive how much they have changed the weapon stats.
Black Ops kinda did it, but not fully.
It's like there has to be a good reason that there are almost no games where you can "create own weapons, share codes and make own special servers", but I doubt there really is.