You're conflating the opportunity cost of having a child with the actual logistics of raising a child.

Cities are by definition economical logistical hubs. It's where the jobs and opportunities are. Most people aren't saying "gee I better pack up and move to suburbia before having a child" it's more like "having a child could cost me my high-paying job so I better not". And that issue is exacerbated in the U.S. with the worst parental-leave policy among developed nations. Regardless though, the trend will still exist in countries with generous parental-leave policies because having a child is an opportunity cost in an urban environment. That doesn't mean the environment itself isn't conductive to raising the child. Completely different stories.