I watched a dog chase a car the other day. I always heard about this kind of thing, but I had never really seen it. Sure enough, the little guy was nipping at the tires of the car as it barely avoided squishing him into the cement. It doesn't make any sense for a dog to think it's appropriate to try to go after something of that size. Dogs aren't notoriously the smartest animals in existence, but they have to be aware of the whole "this thing's easily ten times my size" factor. That's a survival instinct.
So what makes a dog chase a car? It's not like they smell something they want. Most people aren't dragging rawhide bones behind their minivan, so dogs aren't in it for what they're going to get out of it.
I think they chase cars because cars can be chased. Everything else in their lives is pretty still. The ground they sleep on doesn't move. Their food is always in the same place. The toy they play with by themselves doesn't go very far unless they take the time to move it.
It's something about the just unreachable that must appeal to them. It's not the getting it, it's the going after it.
Of course, maybe I'm prescribing far too much philosophy for the actions of an animal that will fall for the fake throw every single time.
But if you ask me why dogs chase cars, I'll always believe it's just because cars are fast enough to be chased.