Disclosure: I've been living in Japan for half a decade now.
There are a number of interplaying and compounding factors that are responsible for creating this situation. I will list a few in no particular order.
- The hours are absolutely insane. My partner rarely leaves her office before 10pm, and by the time she does, she is completely exhausted and unable to do anything other than hit the sack. This type of situation is extremely prevalent and generally affects both men and women. It is no wonder that young people have no time to socialize, go out and meet potential partners when they spend 90% of their waking hours working or otherwise subordinating to this insane work cult.
- Japan is a country of predefined roles and expectations, and at different ages people are fully expected to meet certain benchmarks and seamlessly transition into new roles. One such role is that of a provider, and men are expected to be able to become full-time breadwinners by the time they hit 30. With the economy being what it is and the traditional Japanese notion of lifetime employment being a thing of the past, a lot of men aren't able to live up to these expectations and simply drop out of the dating pool. Some of it is involuntary, since Japanese women tend to maintain very high expectations as to what a man needs to be able to provide.
- Sex is generally highly available and there are all kinds of parlors, services, salons, middlemen and clubs catering to both men and women. It's cheap, legal and safe, thereby creating a disincentive for people to attempt and engage in the old fashioned mating process. On top of that - and this is going to sound completely nuts to those who aren't familiar with the country - there are full-time gigolos, both foreign and domestic, whose only life goal seems to be getting as many notches as possible. They approach women on the street, in bookstores, train stations, malls, you name it. I know a few of these characters and many of them have notches in the 3 figure range and juggle up to 7 women at a time. It's all they ever do, and I suspect that they tie up a huge number of women at any given point in time. I can't say I've noticed the same phenomenon in any other country.
- The local culture isn't exactly touchy-feely and even when people date, it's something they do because it's what young people are supposed and expected to do, not because they like their partner or dating in particular. Just a curiosity I've noticed with many couples.
Obviously, the situation is very complex and cannot be explained with a simple narrative, but I hope this helps clarify things a little.