Just like every problem, there's tons of factors at play. Let's not try to reduce everything to a single variable, because then we are surely mistaking ourselves.

Top of mind, I would say the following factors probably play a role:

- social pressure to conform

- social pressure to be successful

- depression not being very well treated in Japan

- work/school being too much in one's life compared to other things

- lack of support/encouragement - isolation

- fatalism ('you can't do anything about that') kind of belief

But even that list is too short, I feel. I think by living in Japan you grasp a little more what is at play but as I said, it's a whole package, not just a few factors.