It's not and the fixation on Otakus is just acceptable racism. Japans birth rate is falling bc capitalism. You're expected to work all the time and devote a lot of time to work related activities outside working hours. Women are usually punted out of their careers as soon as they get pregnant, losing the limited autonomy workers have and being forced in to the home, while losing a significant amount of income. Cost of living and the cost of having children are both prohibitively high, and the stress of day to day life is such that it's hard to meet people and develop relationships and many people just don't have the emotional energy and strength to develop relationships.

It's the same reason birth rates are falling in the US and fewer and fewer people are in relationships or even having sex, albeit with different culturally rooted specifics. Capitalism is blind to and unable to cope with how it's own inhuman demands on the workers annihilate the very social reproduction it needs to perpetuate itself.