You're baffled people aren't having kids? What bubble do you live in? People straight up can't afford housing much less children. The avg American is completely priced out of owning a home right now. Edit: the replies - y'all are out of touch. Visit an average family in the Midwest with a household income of < $100k/yr that doesn't own a home yet.
Personally, I think it is that expectations rise, both for your own life and for that of children once your purchasing power increases. It's harder to give it up than to not have it in the first place. Having children increasingly has to compete with a lot of other rewarding things that require far less commitment and far lower costs.