The thing is is that most of those weapons are already in the game. The campaign has M60, barrett 50 cal, AK74u, M16 and a lot more.
And they should be put in multiplayer.
Who knows. The reason racing games usually only have 8 people racing at a time at the most is because I guess the xbox can't handle generating 8+ different cars with exhaust sounds and everything. Maybe it's the same thing with the guns.