Not a historian, but working together as a group and not letting the people around you down are values that are emphasized from childhood. Leaving work before the people around you makes it feel like you are the weakest link.

Also there is a lot of weight placed on seniority, so younger people have pressure to work harder to make up for what they lack in experience. While a little less common now, it used to be understood that you shouldn't leave work before your boss.