They already allowed the flag carrier to have a magnum, if they could drive there would be almost no team play. Plus they would be damn near impossible to catch if they had a vehicle nearby.
I can't speak for everyone's experience, but I stay away from objective based game types because there is little team play ever used. Whenever I used to play Oddball, CTF, Disarm Bomb, etc, my team mates would go off treating it like it was a Slayer match. So while the enemy team is winning, I'm stuck trying to do the objective while everyone else is just trying to rack up kills.
I got so frustrated whenever I'd grab the Flag/Bomb and go to get in a vehicle only for my team mates to jump out of it and start fighting the enemy or they just take off, leaving me behind.