I think when making self-protection laws the government probably also weighs:
1. The magnitude of the problem,
2. The difficulty of the self-protection measure,
3. The benefits of NOT using the self-protection measure.
Pretty much everyone knows someone who has died in a car accident; very few people know someone who has died of skin cancer, so i think it’s safe to say that death by car accident is far more common, and therefore the government is a lot more concerned with minimizing deaths from that. The magnitude of the car accident problem is a lot bigger, so it’s more of a concern.
The difficulty of the self-protection measure is probably also a consideration—putting on your seatbelt is so ridiculously simple that mandating it by law doesn’t cause anyone a significant burden. Compulsory exercise, on the other hand, would probably have an even greater impact on public health, but the burden it would put on people is so large that the government is unlikely to consider it.
I would bet that alcohol and tobacco are still legal because aside from the historical reasons using them still has some benefit in the form of subjective enjoyment by the user. Not wearing your seatbelt or not putting on sunscreen doesn’t really have any benefit.
If we are really concerned with public health, I think the real question we should be asking is why we allow portion sizes to be so huge, why we allow fast food to be ubiquitous, why we let law makers be influenced by tobacco companies, and so on.