Here, ‘masks’ refers to the cotton things you see Japanese people wearing over their noses and mouths in pretty much any video you watch of Japan. And the better question to ask is, “Why don’t people in other countries wear these masks?”
Japanese people wear masks for one of two main reasons: 1) They are sick and don’t want to infect others or 2) they aren’t sick and would like to stay that way. I’ve also heard that sometimes women will put on a mask if they don’t feel like doing their makeup, since the mask covers most of your face. This seems like a great idea to me, and a good way of keeping sickness under control (along with hand washing of course). It is currently flu season here in Japan, and many of the students are wearing masks because they’re already sick. I’m considering wearing one for the next week or so until winter break starts.
A second culture note: As you may or may not know, the work culture in Japan is crazy. You are expected to work hard all day, then work overtime everyday, and get home late all the time. The Japanese workday stops for no one. Because of this, rather than taking a sick day so you can stay at home and rest, most Japanese people simply don the mask and head to work anyway. If you use a sick day, you’d better be dying. With my contract, I get a certain amount of paid vacation and a certain amount of ‘sick days.’ In order to use a ‘sick day’ I have to have a signed doctor’s note saying I can’t work, which means I probably have to use one of my vacation days just to get the note.
That being said, the first time I came to school with a mask on (feeling slightly unwell; it was a Monday and I’d been sick over the weekend) everyone FREAKED out. “Oh my gosh, are you ok?” “Are you sick?” “Did you catch a cold?” “Don’t stress out too much!” “Don’t come to class today, stay here and rest!” I tried to explain that I wasn’t all that sick and I could come to class, but my voice betrayed me and came out in a hoarse whisper that only served to confirm my teachers’ suspicions that I was truly ill.