[Yizahi]

Cost of living in those countries is extremely expensive and so is buying any type of housing. New families who don't inherit an empty place to live must work a lot to afford anything, so both partners have to work and that leaves no or much less time for kids.

Second is universal education which rapidly advances, means that women are no longer content with being a nanny plus housemaid forever, and often want to have something more meaningful, leading to no or much less time for kids.

Basically kids require at minimum one adult taking care of them during most of the awake time. And due to various reasons gaps in that coverage can be a deal breaker. For example kids going to day care until 15:00, but parents busy until 19:00. 4 hours gap is not a lot but at early age can be a deal breaker. Etc.

In my opinion we won't reverse this trend until next world war (after such event humanity will partially reverse development and due to loss of life and work, many people would have time for kids). So in the mean time countries need to attract skilled educated immigrants while suppressing housing prices, and the most successful countries in that would win in the long run.