Consider a law that says "you can't drive a vehicle without a license." Is a bicycle a vehicle? What if it has a motor and weighs a lot? A velomobile? What about a hovercar?
There might be 2,000 laws referencing "vehicle [definition #4,849, version 5]".
(Actually, I guess a sensible definition might be based on momentum: we'll call a tricycle a vehicle if you manage to get it going fast enough; the point is that you can hurt someone with it.)