What happens now if you don't have insurance and you need medical treatment? For example what happens if you're in a car accident, do they first ask you whether you have insurance? What do they do if you don't have health insurance & don't have enough money to pay for the treatment?

1) You die? 2) They take you to the hospital and treat you

If it's (2), who pays for you? If the answer is the taxpayers then that's a reason for making not having insurance illegal.

It's the same reason why car insurance isn't optional.