We tore down up to 80% of them over the course of a couple decades. If it isn't designed to house a family of four, the US seems to think it should not exist. So, many of our youth end up living with roommates because they can't afford a family home on their own and there aren't more modest options available.
Edit in response to your edit: You might find this article a good read elucidating the issue: https://www.citylab.com/equity/2013/07/it-time-bring-back-bo...