LogFAQs > #962565922

Here:
https://www.zillow.com/research/housing-stock-islands-of-density-25866/

The answer is NIMBYs. That's why.

Cities and metro areas grow in two ways: By expanding outward into the surrounding, undeveloped countryside; and/or by growing denser within an already-developed footprint. Historically, American cities have done both. But since the 1970s, several of the countrys most economically vibrant coastal cities have gradually been doing less and less of either. The result has been decades of faster-than-average housing price growth in these areas as theyve struggled to add sufficient new housing in the face of sustained housing demand, which underpins their current housing affordability crisis.

Additionally, residential development in essentially all American metros has shifted to a new pattern. Construction in metropolitan interiors now concentrates in clusters of very large apartment buildings, dispersed amid mature, less-densely developed suburban tracts that are virtually frozen in time and produce almost no new housing. There is enormous potential for new housing supply to be had by allowing for even modest amounts of new density allowing a small share of lots currently host to a single-family home to instead house two, three or four units in these otherwise stagnant areas.

Because of local rules and regulations that have accrued over the decades, it is rare today to see the nations low-density suburban tracts redeveloped more densely. Areas zoned for single-family homes are particularly well-insulated from such change, and rarely see the types of multifamily construction common as recently as a generation ago. Such areas account for most of metropolitan Americas land, and comprise the sea of no-growth.