It could be a “cultural” bias in the types of people who live in cities. A lot of young professionals might move to cities because it enables a lifestyle they’l

It could be a “cultural” bias in the types of people who live in cities. A lot of young professionals might move to cities because it enables a lifestyle they’ll enjoy (not to mention well-paying jobs), and that lifestyle probably doesn’t include kids (at least for a period) for many. Whereas someone who wants kids may move further out to the countryside where it’s significantly cheaper to raise them.