Because not everyone has a car but everyone has a body. (And some people have an especially nice body but I won't get into that.)
Also, like car or home owner insurance one can determine any possible loss. For example, if one purchases home insurance against fire but not theft they know ahead of time the maximum loss they may sustain, that being the contents of their home. Also, they can evaluate the chances of theft, say, between living in a gated community with security patrols vs a crime ridden area. Contrast that to medical insurance.
How is one able to make a reasonable evaluation regarding the risk of illness or accident? Cancer? Organ failure? Paralysis?