People come to SF to visit the attractions at the high density parts e.g fisherman's wharf, union square, chinatown, ghirardelli square, coit tower, etc.
Look at a standard tourist map of SF: https://www.sftodo.com/sanfrancisco/wp-content/uploads/2018/...
No one comes to SF to visit the vast tracts of suburbia in the sunset: https://goo.gl/maps/p91HMQTcXBQ2
No one comes to outer Richmond, Sunset, or Parkside for the "charming old houses". It's mostly single-family sprawl.
The funny part is there are 6-10 story apartment buildings in Sunset that are illegal to build under current code. Doesn't seem to be "ruining" the neighborhood.
The vast bulk of SF is probably not of any particular interest, though. Most European cities will have areas of historical interest where buildings are protected, sometimes incredibly strictly (here's a 250 year old 8000sqft house which would require 1.5million renovation to make it habitable, due to strict controls on acceptable renovation methods: https://www.independent.ie/life/home-garden/homes/georgian-p...). But that doesn't mean that you can't densify anywhere. Areas of historical importance are normally a small fraction of a city.
There are very few tourists in the vast majority of the Richmond and Sunset, except near the water and Golden Gate Park (around the museums). Also, there are very few tourists south of Bernal Heights Park.
While we think SF would look great if it were like in the Star Trek movies, that would pretty much destroy a large part of what makes San Francisco San Francisco.