My personal hunch is that this is a form of market failure. If you're a young woman, delaying childbirth in favor of more education and a higher income makes perfect sense, on an individual level. But the more young women take that path, the fewer are available to form communities of care. Consider the economic cost of replacing that network -- neighborhood moms, grandparents, immediate and extended family. Raising a child with one parent working is strenuous, with both it's only possible through expensive and sub-optimal childcare programs.
One typical response is that we should simply subsidize those childcare programs. Fair enough, but the countries that have taken this idea the farthest -- the Nordics -- have still not achieved replacement-level fertility.
Another response is that the earth is too overpopulated already, and we should have fewer children due to climate change. However, we know exactly how many people will be alive in the latter part of this century -- given relatively static mortality rates -- because those people have already been born. Crashing the population after 2100 won't really help anything, and will in fact only introduce more economic challenges and crises for our descendants.