The people living a simple life as you describe have more children than the higher paid inner city folk. Explanations like yours are shown again and again to not explain reality.
But if your fixed costs are much lower (housing etc) then it makes more financial sense to have kids as there's less of a hit from the loss of lots of income (plus expenses, kids are not cheap).
Like, we have two kids and a top 10% (bottom of the top though) salary, and we have no discretionary income or ability to save (very much) after paying for the costs associated with kids and housing. I can completely understand why people don't want to do that, particularly given that 90% of households in my country earn less than us.
Sometimes there is an income problem. Sometimes there is a spending problem. And sometimes there is a cost problem. But cost is unlike the others, because income and spending are individual, but cost is universal.
Inflation from 2020-2024 was absurdly high. This was a universal hit to cost, and everyone felt it.
But outside of that recency bias, Americans have poor financial literacy. Americans have spending problems: credit card debt, car loans, and student loans come to mind. Each of these debt categories aren't inherently bad. Student loans are great if the degree has earning potential, but most Americans don't seek those degrees. Car loans aren't bad if it's a practical and affordable vehicles, but Americans like luxury SUVs. Credit cards aren't bad if you buy a washing machine to save on laundromat visits, but Americans like expensive vacations.
Americans also have problems with income, but that's even more complicated.
