If you have something to lose, you will be far more cautious about all aspects of life than someone with nothing. Sex is one of those primal drives that are easy to ignore when there are other things to pay attention to, and easy to indulge in without thinking much when everything has gone to shit. It's not exactly hard to understand that people with options will plan their lives much more intensively than those who don't know if they will be able to eat tomorrow.

Reproduction can be said to be incentivized in low-income groups as children can often participate in the tasks (typically manual labor) performed by the family. Meanwhile, in high-income groups, children take away resources that could be invested into the parents' career progression and wealth management.

Of course low-income groups also suffer from lack of access to contraception, so it's not as straightforward, but that is the birds' eye overview of how it typically works.

Back in the olden days when the majority of human labor was agricultural, having more children meant more hands to work the farm, so having more children actually improved your resource situation. Humans on some level have a biological drive to procreate as well, but if the incentives against having children become strong enough the humans will prioritize short term survival, or maintenance of their standard of living which can feel like survival if slipping for even one month means homelessness.

Free time might be a prime causal factor here. Both the wealthy and working poor may have too little leisure to achieve above-replacement birthrates. That would explain why there are places on Earth with high birthrates yet low GDP per capita.

I'm also suspicious that "wealthier" countries are subjected to more substances which disrupt fertility than poorer countries.