This is all the fault of selfish young adults. The more women are educated and able to work and pay for their own lifestyles, the less they want to become moms, which is a very specific and limited role. Why do that when they can live how they like, travel, buy luxury goods if they want and generally live comfortably.
Japanese men, on the other hand, don't particularly want to deal with women who don't exactly need men and don't want to get married any more than the women. Like single women, single working men are free to indulge their hobbies and games or whatever else they want to do. The have jobs sufficient to fund it.
For both sides, getting married and having kids represents a loss of income, but even worse, they lose tremendous amounts of freedom as they are forced into extremely narrow roles for mothers and fathers. Moms will be expected to do all the childcare and keep house and home, while the dads are expected to work ridiculous hours and turn over almost all their income to the wife. She becomes a housewife mom robot. He becomes a work robot. And that's IT.
These are smart people. They see how they were raised. They see what will be expected of them and want nothing to do with it. They are the generation that can say NO.
The government talks big about meaningless changes that do nothing at all to address why young adults are not having kids. They don't want to make the real changes it would take to fix the problem. At this point, the only way to save the population would be to outlaw not having kids.