Because it inevitably butts up against people’s desire to not live near lower socioeconomic classes who may not be able to afford a car centric life. See the prevalence of school rankings (proxy for parental wealth) in real estate websites.
Older cities or older areas within cities with dense construction were grandfathered in, but I doubt we’ll see another new city like that in the US. The primary way we separate us from them is with unwalkable distances.