I'll agree that a vehicle wouldn't serve any useful purpose to an exploration/savaging game. The roads don't take you to nearly as many locations as there are, and the land isn't designed for vehicles.
The cars are obviously nuclear powered, based on their explosions and the leaking radiation after the explosion. However, they probably are dead from the EMP of nukes going off in the area 200 years ago. Any working vehicles would probably have to have been built afterward, where supplies and experience would now be at short supplies.
Gas would have been destroyed in the heat of the blasts. Any that survived would have been used up for generators, which we see in the game here and there. Power is more important than a car, afterall.
Building a fusion or fission car would probably be impossible in the wastelands. I mean the people of Megaton couldn't even disarm a nuclear bomb. No one knows how to build a nuclear car.
It's possible someone could build a electric powered car, but again, you'd rather any batteries used for weapons or running machinery for food and water.
In the end, most people don't need a car because no one is in any rush to get anywhere. Everywhere sucked, so you just stay where you are and make a meager living there.