Statistically, like many of the other developed countries, people tend to have fewer and fewer children. Having more children and raise the next generation is the basis of any species. It's the primitive urge.
But you should know after developing a country, people start focusing on the other aspects like their own careers, entertainment, and so on.
Plus, people outside of Japan tend to see Japan as a "weird country" and bend facts.
For example, the marriage rate of Spain where the previous answerer is from, is much lower than Japan.https://worldinfigures.com/rankings/index/217
Also birth rate of that country is 1.33. This is also lower than Japan (1.44). Still they talk about the "Japanese low marriage and birth rate" without comparing it to the other countries. This shows there are many who can't see as a fact as a fact. They are living in the "facts" which I call "fantasy".