Maybe SF really is a parallel universe. Here in the UK, basically anyone earning a tech salary can afford to live within 10-20 mins cycle of work, even if they want children.
You won't have a home with a garden and all the rest of the knick knacks, sure, but that's a different discussion.
The reason why they don't is still due to the high rent though. When living with a roommate or two, or living a bit further away means you get an extra $1000+ a month to spend on other things, that can be very enticing.
For example, living in Daly City would give you a ~30min commute, and the average rent there is $2,500, so $30,000 of post-tax income annually. High, obviously, but certainly doable on most SF dev salaries; Hired says the average there is 145k gross, so perhaps 100k or so after tax: https://hired.com/page/state-of-salaries
One good reason to leave the city would be if you prioritize renting a house with a 6000 sqft suburban lot, which you can’t find in the city. Another would be if you have kids and want a good school district.