The poor have nothing to lose. It's the same around the world today, poor countries have high fertility. The OECD middle class has low fertility because they're worrying about other stuff (lost wages, career stagnation, etc). The rich (high fertility) have plenty of money for private schools, nannies, housekeepers, extracurriculars, etc for 14+ kids (Musk).

It's weird to say the poor are more secure than the middle class, but that's what the data shows. Opportunity cost is a real thing. If other middle class people forgo kids and you don't, house rents will go up and you might not be able to afford a place.