It's a good question. Wikipedia tells me that the rate of deaths per 100K vehicles is about 14 in the US. As a point of comparison, selling raw, unpasteurised milk is prohibited in most UK states. It has a death rate of about 1 per million drinkers.

Of course laws are not always rational, but somewhere in there someone has decided that the death toll/benefit trade off for cars is worth it and for raw milk is not. It might be surprising, but I actually would rather have it the other way around, but I think there is considerable room for reasonable disagreement.