the servers aren't designed for vehicles. They'd lag out. Simple enough. It's different style of game play too, there's not alot of interactment with the environment, if you haven't noticed, in CA. The game isn't really designed for stuff like vehicles. There's very little in the way of doors and breakable objects, except for stuff that's extremely scripted. There's no real ragdoll effects, which means every single thing for vehicles would have to be pre-scripted, otherwise they'd have to remake the engine.
Now for those of you who have never done programming for games before, this is NOT a fun or easy task. Specially when the game is for the most part, free.
I wouldn't do it if I were them. Too much work, no real gain.