Presumably not all the cars were abandoned at the same time. It's just that our characters always stumble upon a vehicle that's still usable because it was only recently abandoned.
For instance, some of the walkers at the motel where Merle hot-wired the car may have been recently using that car, when they succumbed to something or other.
or not. Every vehicle they try has a live battery in it. You would think once in a while they would get in one that has a dead one.