That's hard to know, but keep in mind that the majority of renters in the U.S. do own cars and in most cases, personal auto policies do cover you for liability when renting in the U.S. so it's mostly applicable to those people who don't own a personal auto and thus have an existing policy.

Obviously the answer would be significantly different for residents of say New York City where most people do not own a car because it just doesn't make sense to.