I can't speak for SF or Seattle, but I suspect a big reason for lack of families with children in many large American cities is that, for the most part, public schools are unacceptable for any family that cares about education.
DC, Philly, Baltimore, Cleveland... all the same story. Even in gentrifying and resurgent neighborhoods, parents with the means to do so leave as soon as their kids get to school age or enroll them in extremely expensive private schools. Some might try charter schools or selective magnet schools, but the local neighborhood school in your typical big city is typically a hard "no go".
DC, Philly, Baltimore, Cleveland... all the same story. Even in gentrifying and resurgent neighborhoods, parents with the means to do so leave as soon as their kids get to school age or enroll them in extremely expensive private schools. Some might try charter schools or selective magnet schools, but the local neighborhood school in your typical big city is typically a hard "no go".