Sadly, this is a result of deeply-rooted parts of Japanese culture.

In Japan, you don't just walk up to people and strike up a conversation. Foreigners might, but native Japanese largely don't.

To establish some sort of repertoire, you need to share some sort of acceptable social context with that person: went to school together, played on a sports team together, work together, etc.

And, given that Japanese companies commonly have working hours that would be classified somewhere between "insane" and "criminally negligent", you don't really have time to socialize with anybody outside of your office.

This all goes triple for Tokyo, which has a very cold and sterile personality.

Expats tend to glop together into a tight group -- the "foreigner" community in Tokyo is like a small town in and of itself -- but for the natives, the end result is that there are a lot of very lonely people here.