The Japanese are by nature and culture a more industrious and introverted nation. I've actually watched many documentaries on this subject, the startlingly low birth rate trend in their country, and I think some of the issues they've had for a while now are starting to become pretty common in western culture. There is a high expectation for people to succeed, particularly men, and due to the sheer pressure they work and study to the point where it becomes detrimental to their emotional well being.
As you mentioned, many of them have truly gone their own way and never even leave the house, they even have a specific term for them in Japan that I don't recall off the top of my head.
Let's face it, if you grew up feeling that nothing was good enough and being put under such pressure in school, and later work, you are going to have issues with self esteem, possibly depression, anxiety, etc. You go and work tirelessly, grinding every single day in a position that probably doesn't offer much in regards to personal satisfaction. How is such a man expected to be able to do well in his romantic life if that was his life experience?