It is because of the culture of the Japanese people. In the World Cup and other stadiums they pick up all their trash when they leave and leave it pristine. They don’t have to worry about fines in the stadium.
They are rather polite, but they also have good residential trash pickup.
In this case, anyway. It's actually pretty difficult to get rid of some things, so people can build up a lot of junk at home - Marie Kondo's books in Japan actually have extra sections on how to get rid of it.
https://www.usatoday.com/story/sports/sportskind/2022/11/28/...