Which takes less storage space, a rack of batteries or a rack of cars? Especially if you need to accommodate big car/small car/smoking/nonsmoking/kids/no kids. And take the time to clean every car, because you'd better believe that would be legally mandated pretty fast. If it isn't already. So that's probably a bigger investment in labour than just "some guy to switch the batteries."
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"