Except there’s too many wearing them for this to be the only issue.
I’ve been to Japan/Korea many times and it’s certainly not just because somebody is sick and is being considerate.
I’ve wondered if it is partly to avoid germs of others, air pollution, as well as being sick themselves. The thing is, I read an article that stated wearing them isn’t really healthy. The moisture from breathing creates a surface that is perfect for germs to develop. I’ve no idea how often they replace them.
Also, their group eating habits include chop sticks and sharing of food from the same bowls. This flies directly in the face of a pure “worried about germs” theory.
Air pollution is also a factor, but it is predominantly a matter of consideration to others. Whether they are effective is another matter. Peer pressure overcomes logic and reason.