Geopoliticist Peter Zeihan has a pithy explanation: "On the farm, children are extra hands, a kind of wealth; Crammed into an apartment (either in a city or on a collective farm) children are expensive, loud conversation pieces."

That said, one of his points is that the US is gradually turning away from the Bretton Woods Accords of ’44 (US both opens its consumer market and protects everyone's blue-water international shipping on its own ‘dime’ - AKA the "Global Order"). This plus antibiotics plus vaccination plus Haber’s process to produce fertilizer from atmosphere plus gobs of energy has led to a population boom. Kids live into adulthood, marginal farming areas with chemical inputs become productive (e.g., Brazil). This has made Malthus wrong — so long as the ‘Global Order’ continues.

If/when we do finally turn away (it's been happening since Reagan - with a 'hold' due to the Ukraine invasion) the earth's population will drop precipitously (as in famine and even decivilization). I applaud the growth rate going to 'stable', but a crash is worrisome.