If sunscreen is supposed to provide specific health benefits, namely to reduce cancer risk, then it is a drug, not a cosmetic. Regulations should ensure it provides the intended effect without undue harm. Cosmetics are given more leeway because they are, in principle, neutral from a medical perspective. Why would you want to treat a cancer related product like that? Saving upfront time and money, at the risk of having to spend a lot more time and money later in healthcare, is not a good reason. If anything, we might head the opposite direction. Some people think we should start regulating dietary supplements as drugs rather than food.
FDA does regulate cosmetics components, and FTC regulates what claims they can make. It’s hard to make a binary decision that one thing is a drug and something else isn’t. You can appreciate there is a spectrum of side effects with moisturizer and sunblock on one end, supplements in the middle, and chemotherapy drugs on the other end.
Personally, I think that Americans simply don’t treat skin cancer as seriously as they should, and so the market has not provided more choices.