Subways[1].
Sure developers would love to build more chicken coops for tech bros. But they absolutely don't want to kick down coin to pay for the subway system needed to support that density.
[1] Oh yea SF has BART which was built 50 years ago and not expanded since. And the central subway which is 1.7 miles long, not open yet after ten years of construction and 20 years after it got the green light.
Now, it's trendy to view any investment by democratic government as wasteful, and to reduce the taxes by which the community makes that investment to nothing.
What will happen to future generations, who have us as their forbears rather than the visionary New Yorkers.
It was only later that all were merged into one publicly-managed one.
Well, yes. And deep bedrock and fairly stable geology that makes them possible.
Snippets I've read over the years seem to say that subway tunnels and deep foundations don't tend to fail in earthquakes because they to move with the ground around them. And because they are already structurally strong.
Basically the lack of mass transit to support higher density is because of lack of willingness to pay for it. Unwillingness to pay for public infrastructure is due American economic ideology. Not anything specific to San Francisco.
https://www.businessinsider.com.au/earthquake-expert-san-fra...
I'm going to push back on this. Manhattan is less than half the size of San Francisco and has nearly four times the population density. The Bronx is roughly the same size and has double the population density. It isn't that San Francisco can't expand, it is that it doesn't want to.