That's because the poorest people are also the ones having the most sex, because sex is a cheap form of entertainment. Poor people are also the least able to afford birth control. More sex plus less birth control equals more children. No one, however, would argue that these children have the brightest future with the most opportunities ahead of them.
It's kind of like wondering why people don't want to fall into puddles. And you notice that poor people who walk to work in the rain are the ones falling into puddles the most.
It's not about money. No one in history has ever had kids because they suddenly felt financially secure enough to. No one wants kids. Across the globe, when the standard of living rises, fertility goes down. Reproduction is a biological Ponzi scheme. When you're given an out, you take it.
More money makes life more easy, but I get the sense that money has little to do with birth rates. After all, the poorest people in your city, state, country, and planet are the ones having the most children.