The real reason: people in rich countries have been tamed to be orderly cells inside larger and longer-lived organisms -- the nation, the culture, a religion or class or movement, a corporation, whatever.

Within a larger organism, there's no surprise individual cells don't wildly reproduce in the organism's warm, safe, nutrient-rich environment. That's against the conditions of formation/inclusion. When it happens, it's often a cancer that threatens the whole.

what do rich cultures have in common? they are civilized enough to get rich. they have good enough values that let them cooperate peacefully with others, etc

the sort of good values that make societies richer include caring about your children individually and spending much more money per child, instead of just trying to have a lot of kids and use them for cheap labor.

also, our culture considers children a pain in the ass. then people wonder why we aren't having as many as we used to? because more people are doing what they want instead of blindly following tradition.

some parts of our culture think that. Other parts think children are something with a high cost and an immeasurably high return in happiness! :)

umm, yes. a high cost. not just in money, but in "hassle". our culture, almost entirely, also thinks that kids have a high return. yes. i wasn't denying that. though i do think that set of opinions constitutes the wrong attitude to have.