In pre-industrialized nations the amount of resources it takes to raise each child is minimal. People have so many children because the resource requirement for each one is minimal or even a positive addition to the income of the parents.

In industrialized nations that math reverses. The amount of resources each child requires to become a functioning member of society increases dramatically (up to 10,000x more). Children are a pretty much universally a net cost to the parents not a source of income.

The birthrate is Japan and S. Korea have plummeted recently because of extreme wealth inequality.

S. Korea - the bulk of the countries economy is controlled by 5 families. The average 20 year old is in debt, working obscene hours per week and is barely making ends meet.

Japan - Has had 30 years of wage stagnation as the wealth inequality has steadily been growing. Young Japanese face the same issue as the S. Koreans. Long hours of work for little pay and no benefits.