Probably because a lot of those people bought the same house for $600,000 10 years ago. Their monthly cost is likely less than rent.

I know someone who bought a house 5 years ago and rented it out. She had to cough up $1,000 per month to pay all expenses. Rent has now caught up so she's breaking even.

In the rest of the US where housing prices aren’t as inflated, it can actually be cheaper to own.

It might possible that most of the people in SF who own property have owned it for a long time. So for someone buying a house now, they would be paying a ton of money. If you bought the house more than 10 years ago, when prices were much lower, your mortgage would be much more reasonable and the monthly costs you would be paying would be less than what people are renting for currently.

Hence complaints of NIMBYism. The lack of new development in SF drives housing and therefore rental prices upwards, and so if you owned property before the bubble, you can now charge rent at the modern, high rental costs while paying a mortgage based on the lower pre-housing crisis prices. You could easily be pulling in thousands of dollars a month this way with a single rental property. So lots of people are now have a big incentive to fight against new development.