"If I didn't have children, who'd take care of me when I get old?"
"If we didn't have children, who'd work for our pensions and keep society running when we retire?"
"I want to live a happy life after I retire, and you (young people) are obliged to provide that."

Real words I heard.
A lot of people have kids mostly for future-proofing themselves.

India: "I need many children to support my everyday life and me when I'm old."

Germany: "wtf are children?"

(A bit exaggerated of course, but should illustrate your point.)

Access to opportunities and birth control drop birthrates.

Lots and lots of poor countries have large populations because poor parents are hoping many children can work. Also lack of access to birth control and far right groups insisting children are a religious necessity.