Besides, leaving at one's parents is not equal to be dependent, it is economically sound and young people who have entered the work force may live in the same place as their parents but few share much more than that.
You can totally see this while travelling. Very few japanese youngsters travel abroad and even fewer do so alone.
This is in contrast with say young Koreans. I meet those in droves usually either alone or groups of 2/3.
I don't understand what happens between childhood independence and hikikomori but something definitely happens.
Granted that's for a relatively tiny space, but still, not that expensive, and that's probably the most expensive metro.
I don't know how Tokyo's market compares, but I wouldn't be surprised if there are some different dynamics at work.