After spending 8 years in your room and only talking to your parents through a door, it probably takes a huge toll on your mental health. And since Japan is a shame/honor-society, the effects compound very quickly. To make matters worse, once you fall out of the “right” track in Japan, you are categorically excluded from certain forms of work and social status.
This means that if, say, you stay in your room a year after high school instead of going to college, you will never be accepted into college for the rest of your life. You will never be able to get a high status salaryman job. This leads to a mindset that there’s no going back and that the rest of their lives are ruined.
The hikkimori are a class of people that suffer from a culturally specific syndrome that would manifest itself in other ways outside Japan/Asia. Depression or anxiety probably.
I think a large part of the problem is that it is very common in Japan for children to live with their parents for a relatively long time compared to Western (or at least American) society. And since Japanese culture is built on the honor/shame duality, everytime the child disappoints their parents, the effects compound. This eventually leads to a point where the child is so ashamed, they just lock themselves in their room to escape.
If children just left the house earlier, I think this would solve a lot of problems. They would leave their rooms because no one would be there to judge them.
As an aside, it’s interesting that the problem is only with men. Women are held to a lower standard and don’t feel the type of pressure as men to become successful and honor their parents.