I know a family who still wears masks in public because their daughter is very sick, very weak, and they don't want to risk her getting covid. Perfectly sensible. I also know people who wear masks in public because they're a little nuts... just can't let go, or politically or psychologically hung up.

So what can you say? You see a stranger wearing a mask, you don't know if it's for a good reason or not. Probably the odds are that it's an emotional or psychological hangup, but it may be for a really good reason. So you need to just shrug and let people do what they think best for themselves. They're the ones responsible for themselves.