Why is there a declining birth rate in countries like Japan and South Korea?

Because parents want to have boy children instead of girl because they get the advantage of their child getting married and taking the wife which gives them more money instead of having a girl. when the girl gets married the parents of the girl dont get the advantage of getting more money.

Because those places want a secure population and don't want to over populate so u are only allowed to have 2 kids I believe but that always change plus kids go to war and kids are getting killed