It's the opportunity costs I think. When you have no status to lose, no luxuries to miss, there's not much of an opportunity cost.

Opportunity costs are at least what I think starts the trend. After that, it probably mimetically compounds, eg are there even family sitcoms anymore? How many influencers feature children prominently as part of their lives?

Then begins economies of de-scale, where child rearing is more expensive and burdensome because it's uncommon and there's not many social and market forces anticipating children ... and then having children becomes the luxury.

I think it's not just that. You just don't need children in more developed places.

In poor countries, children are your pension insurance. And having a lot of them makes sure some might survive to take care of you when you're old.