Work hours are long and grueling. Childcare is phenomenally expensive, and that’s assuming you can even find childcare, which is near impossible. Children are expensive. Cost of living is high. Cheating and domestic violence are rates are high. Women have no confidence that they will be supported or be able to provide support if they decide to have a child, and they are very right to think that.
Scarce resources avaliable for women who have children is the reason for the decline. You could never pay me enough to ever have a child in Japan.
Not mention, plenty of Japanese adults grew up being abused and/or neglected by their own parents while growing up in a pressure-cooker society. Maybe they’re thinking twice about bringing another human into the world because of that, as well?