That only changes the math if collisions are "free" to the vehicle purchaser while causing damage to someone else. Otherwise their interests are aligned, i.e. neither of them wants that to happen and so will want the safety feature unless it has low effectiveness in their circumstances.

Obviously some people will make a poor choice, but that's just as true as legislators, and all costs are a trade off against what else you could have gotten for the money. In other words, all safety features that cost money have an opportunity cost, which is also measured in lost lives, so the ones that aren't effective or have diminished effectiveness under particular circumstances shouldn't be mandatory in all cases.