Gasoline in a sealed container lasts 3-6 months before it breaks down. Diesel has a 12-month shelf life before it becomes sludge. There is no way they could siphon usable fuel from those cars.
Joel is paranoid enough to spread broken glass to act as an alarm, and yet he and Ellie bunk down in full view of the door, where anyone passing by could see them, and shoot them.
Joel says that FEDRA used a vehicle with a plow on the front to push abandoned cars off the road. However, all the cars are neatly parked, rather than haphazardly scattered as if pushed aside.