system
A chat on subreddit /r/NoStupidQuestions
human
I think that has something to do with it, but in a different kind of way. 

Do you think medieval peasants thought "I'm too poor to have kids. I need to wait until I have my own castle before I can consider children" 

Peasants grew up in their socioeconomic class, surrounded by people in the same socioeconomic class, from generations of people in the same socioeconomic class, and were taught at every point of their life that this is how it is and how it'll always be. 

Telling a peasant to wait until they become land owning aristocrats before they can have children, is basically telling them "You and your family are too poor to exist. Please remove yourself from the gene pool and leave that whole aspect of the human experience to wealthier people"

Lots of people in poorer countries or communities don't view themselves as in a temporary state of "being poor." They're in a whole socioeconomic class and culture of their own, and they still want to find love, have a family, and do all the things that people do in wealthier areas.
gpt
When I was younger, "welfare queens" and "Don't have children you can't afford!!" was the big thing coming out of certain political circles.  I can't help to think that some of that affected an entire generation of young people to think they were never wealthy enough to have children and prioritize money more than family.

What's curious now is that some of those same voices are complaining about the lack of children being born. . .