Why the fuck does anyone care if someone else is wearing one

It's because all people are emotional first, and many people are stupid, too.

Seeing a mask makes them think about the pandemic, disease, mortality, death, all that heavy stuff. That feels bad. Rather than deal with that in a healthy way, they can just lash out at the thing that's bringing the bad-feels to the foreground. That is stupid. That's yelling at the smoke detector instead of doing something about the fire.

Accepting that wearing a mask is a reasonable choice means accepting a bunch of other uncomfortable things.

This can also go down the "in group good, outgroup bad" route. Belief is social. People do shit to feel like they're part of the group. Somehow wearing masks became political.