The real reason is the same in Japan as it is in Europe and America: even controlling for hours worked, low wages, high cost of living, and other economic costs associated with having a child, women are choosing not the have children because it would require them to end their careers in their prime. It is extremely difficult to have a child and a career at the same time as a woman, and many never return to the workforce. Even women who make good money and can afford to have children decide not to because it would require them to stop working