What happens in Japan if you put in a simple 40-hour week, do good work, and then leave at the end of the day? I also understand their government makes it very difficult to fire employees. And I can't imagine they'd reassign you to watch the paint dry if you're actually doing good work. Would you mainly miss out on promotions?
Is it just a social thing, or is it economic? Since it's harder to fire people in Japan, there's a lot of risk to hiring a new employee. So maybe the culture helps weed out uncommitted people, which helps derisk hires? Or, to a certain extent, lets you get more productive hours out of them.