Do they? I mean, even if first-world people aren’t as well-off as they could theoretically be, they’re still much better off than people in poor counties (or their own ancestors a hundred years ago). But those people in poor countries and those ancestors have/had a lot more children. Meanwhile people in Sweden have fewer children than people in the USA.
I think that many people in first-world countries do not in fact want children.
(And within a country, poor people have more children than rich people, so actually making more people poor would increase fertility.)