This is all post-facto rationalization to avoid a more glaring truth.
Women aren’t having kids - because they can avoid having kids. While still having sex and having societal benefits, including retirement. That is a very, very new and unusual situation when looking at the span of human history.
Birth control is a pretty new invention (mid 60’s approximately), and societies globally have been steadily rolling it out/making it available more easily.
Childbirth is dangerous (especially the first time), usually ruins one’s figure (and hence ‘market value’), and is economically very expensive.
At a minimum 4-6 months of ‘non productive’ work time per pregnancy, not counting the several years of near constant caretaking for a newborn. Which kills normal careers, where taking ‘time off’ pretty much guarantees you’ll be surpassed by all your competition - and 2-2.5 years pretty much guarantees it. And that is for an office job, not labor.
Before birth control, the only way to do that would have required celibacy or constant abortions - and good luck suppressing that drive for most people!
It’s like wondering why almost everyone is fat and out of shape when there is no public transit, everyone has to drive everywhere (and gets stuck in traffic), food is terrible and junky, jobs are mostly sedentary, etc.
Of course that’s how folks will be on average when that’s the environment! And birth rates are by definition based on the average behavior.
Coupled with the fact it’s relatively easy to survive/live in society without kids now, and it’s a double whammy. At least until the consequences of having no kids for a long time come home to roost - then everyone is going to be fucked. Figuratively.