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.
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.