Have a kid in the big city is the ultimate flex. No one else has kids because they can barely afford to live there themselves, let alone support a kid in it.
Edit: Also, it's not as if the author failed to mention that it's expensive to raise families in the city. It's that he says "yes it's really expensive to raise kids in the big city" but then goes off on a completely different tangent to say that everyone living in NYC and SF are "affluent childless" singles who just want to turn the city into their own childless entertainment machine, which completely negates his earlier comment on cost of living as the primary reason why there are so few young families in the city (plus something about fewer people enjoying sex).