Children are expensive. Children take away a lot of your otherwise "free" time (although spending time with them can hardly be considered an objective "waste"). Grandparents don't want to put in the time either, they want to go on cruises and fully "enjoy" their life (along the paradigm of "what can this really do for me?").
Ultimately we end up with parents who want to offload their kids, stuffing them into (expensive) daycares earlier and earlier (despite what research says are negative effects of such early child separation). We have lost the traditional support system of older generations being around, living with new parents, and helping in a meaningful and impactful way with child raising. We have prioritized large inefficient housing, mostly as an investment vehicle, and have discounted the less economically-linked investment and rewards that come from raising children.
In any case, this is a perfect storm for dropping fertility rates, and it seems at this point these factors are so ingrained in developed economies that it can be considered a natural consequence of a population who has pivoted towards wealth building and economic priority.
Despite this shift towards selfish economic efficiency, is anyone really happier? Really really tough to say.
I can say that despite the cost, the career impact it had, and the completely changed dynamic of how I spend my time - raising kids has been the highlight of my life. I've never really been one for considering pure economic and personal efficiency though, and I've noticed I'm more and more of an outlier among my social circle.