The natural set of incentives does not work well with human psychology. It does not prevent mishandling of motor vehicles, or at least fails to prevent it in enough cases that additional disincentives are needed, to ensure the safety of the public.
Stated another way: Enough people are bad enough drivers that we need laws and civil liability to create additional incentives against bad driving. The threat of collision, property damage, injury, or death to oneself, passengers, and people outside the vehicle is clearly not sufficient.
Driving is actually a great analogy, but maybe not for the reason you intended. If we relied only on individuals to act responsibly, the roads would be much more dangerous than they currently are.
