I must note; That explanation is just some stupid shit I made up. No, I don't believe it at all. I just made up some exaggerated explanation, because I hate it when major stuff is just hand-waved as "magic". If this makes your scientific nerves die, then... Well... Live with it.
Also, cars are much easier to draw without a roof. That's the only reason for why Doc drives a cab. Lazy me, but it's kinda cool on the other hand.