that...is the point of the whole thing...
Driveby is supposed to be one of the most powerful attack/defense perks in the game. It can save you a lot of times when you fight outmanned, also it can ruin you if you are being driveby'd.
Besides, if someone jumps on your roof (which is highly common), he can damage you with any weapon and force you to leave the vehicle or drive away fast in order to have him dropped.
Big -1