maybe 1/2 of my friends from college live in SF. I have a huge social network here. Objectively the city is not awesome, but I can't imagine moving somewhere else and starting from scratch. Lots of tech workers are in this situation IMO.
If you went to a top school (top 25 - 50 in USNWR), then your social circle was much more "mobile" than average.
At this point I would be somewhat saddened to move away from a network I've been building for ~15 years.