Because the fatal flaw of a lot of advanced societies is elevating individualism. You are ultimately just a part of the human species and we evolved naturally t

Because the fatal flaw of a lot of advanced societies is elevating individualism. You are ultimately just a part of the human species and we evolved naturally to produce babies and raise them (i.e. desire for sex). We were also a lot more tribal and it was easier to raise kids with a village. Combination of the latest trends of the past few decades (contraception, individualism, nuclear families) goes against the natural design for the sustenance of our species. Nature doesn't have morals and is pretty ruthless, so if a sub-species doesn't want to have kids, it will cease to exist and will be replaced by the one who values kids. South Korea will be an interesting example to watch - whether they reverse their course and start valuing kids as a society, or they might get replaced by immigrants or are taken over by the neighboring countries. Nothing good or bad inherently with either option because life is meaningless anyway.