You are questioning why, out of all dangerous things, we have seatbelt laws, but we don’t have laws against things that are equally or more dangerous than not wearing a seatbelt.

My answer is that you are looking at it backward. Seatbelts are legislated because way more deaths occur from cars than from contrived alternatives like skiing over toddlers. It has nothing to do with comparative danger or individualism. It’s about scale and aggregate social impact. If toddlers were getting mowed down by the tens of thousands per year, we would have specific laws against that too.