That’s because people pulling a nice paycheck have gotten a taste of stability and don’t want to risk losing it, and this is intensified when the economy is turbulent. People making less never had stability in the first place and don’t have as much to lose.

Aside from that, it's merely observations/anecdotes, but from what I’ve seen people who have managed to achieve a massive uplift in economic status (say from minimum wage in their mid-20s → net worth north of $500k-$1m in their mid-30s) are more likely to have children than people who’ve always been wealthy. I would theorize that such individuals feel a greater degree of economic freedom, having lived at the bottom and being able to make more effective use of what they have.