Because densifying a major metro that's already pretty dense is totally logical. The rest of the Bay Area should be much denser too, its 83% zoned single-family. However they have a much longer road ahead building infrastructure that SF already has. Putting the density with the rest of the density is totally rational.
Brooklyn has 38,000 people per square mile. Queens is 22,000 people per square mile. Manhattan is 73,000 people per square mile. Hoboken is 48,000 people per square mile.
SF is not dense. SF should be the Manhattan of the Bay Area and instead it's got the density of like East LA.
The problem when people pull this stat is that we should really be comparing the density of the Bay Area to other major metros - not just SF. SF is a tiny little postage stamp. It's much more comparable to one of the NY Boroughs. It's just that SF city/county is gerrymandered out of the other peninsula counties.
When we do the logical and look at the SF-SJ-OAK MSA it has a population density of 700/sqmi, as compared to New York City at 29,302/sqmi. It's comparable to Augusta, GA, or Norman, OK.
If we compare at the densest are in the SF-SJ-OAK MSA, San Francisco at 17,000/sqmi - to New York City's Manhattan at 73,000/sqmi - the picture looks awfully different.
SF is woefully underdeveloped, and the MSA is devastatingly underdeveloped.