Most people are biologically wired to want children. "Survive and reproduce" is pretty much the driving motivation of all living things. Most children weren't conceived as a carefully planned retirement strategy. No cost/benefit calculation is required to convince most people to have children, but you can certainly force them into a position where they have to start thinking in those terms. We've just hit a point where societal and environmental factors are discouraging people from doing what they'd normally do.