Probably because the subject attacks the fundamental instinct to procreate. How do you tell a couple that they legally can't have children? Many of my friends define their lives around having a family, it gives people a deep, and rewarding purpose.
Speaking as bluntly as I can here : if you want to have kids to gain a 'deep and rewarding purpose' because you somehow feel lacking in purpose, please don't have children.
It's analogous to a teenager that joins the military out of high school because "they don't know what they want to do.".
Figure things out, then make a decision. Don't make some big brash decision in order to figure things out. The only 'depth' that might added to your life may actually just be unenjoyable complexity and nuance.
This is a fundamental tragedy of the commons. I feel guilty about even wanting to have two kids at some point due to the climate impact. Everywhere I look I see people having large families without caring. This has to be balanced long term for our species (and others) to survive centuries from now.
https://www.gapminder.org/answers/will-saving-poor-children-...
https://www.gapminder.org/tools/#$state$marker$axis_x$domain...