Maybe people thought I was joking? I wasn't though. Sun damage to the top of your head is a serious concern in being bald. It puts the lie to the statement that the only reason being bald is bad is that people are prejudiced against it. No, it's bad in its own right. Hair is useful protection for your body and the top of your head is the part of your body that is the most exposed to the elements.

I don't really see how you could have sun damage underneath a hat.

Always having to wear a hat is worse than just being able to have hair (if that's going to be an option). Hats are limiting fashion-wise and not appropriate or possible in many situations. The beach is particularly problematic, for instance.

Finding and continuously wearing a hat that would entirely compensate for a lack of hair is non-trivial.

Driving at 70mph down the road in a Jeep with no top and no doors, e.g.

I guess, but even if you have hair your face is pretty vulnerable to burning.

Not as much as the top of your head, though. And regardless, even if your face burns, at least your head won't. The more sunburn you get across a larger percentage of your body, the worse it is. Getting less sunburn is always better than getting more sunburn.

Are you a bald, active person? If not, I suggest that you lower your baseless skepticism and open your mind a bit.