That's all hip and cynical, but I'm pretty sure motor vehicle insurance would be a good business even if states didn't mandate that people carry it.

It'd be more expensive for the people that did carry it, but it'd be a large business.

And of course, in many states there is the option to post a bond to prove ability to compensate.

That only helps as long as there aren't many insurance companies. If there are many insurance companies, competition between them drives the costs down to marginal cost plus normal profit, in theory.