Anyone looking at having children (or any other relationship really) through the lens of cost/benefit will arrive at the logical conclusion that it's a huge investment with an uncertain reward, so likely not worth it from that perspective. There's no arguing with logical reasoning.
People in the developed world(or adjancent) have children anyway because they allow themselves to not apply such thinking for once.
Many people with children talk about how much more wonderful their life has been since they've had them (including here in HN comments). If you think there is a good chance of that happening to you, that's the "benefit" in the cost/benefit analysis and having children can end up being the logical conclusion.
I assume you meant developing world, in which case the reason is probably that many women do not have the opportunity to apply such thinking, due to lacking financial independence and/or access to highly effective contraception such as IUDs.
Nope, developed, where people have children despite a culture of analysing everything to death.