The weirdest thing about SF is that there are no kids.
I think it gets magnified in the wake of the unreal inconveniences of large metropolitan area's housing--traffic--etc. You, HAVE to think that yes, you're getting something for that inconvenience that you can't get anywhere else (whether it's true or not), otherwise, you simply wouldn't be there.
I experience flaking a fair amount here in the Denver/Boulder area as well...
For instance, if you have a standing appointment for Game Night on Wednesday, and don't show up half the time, someone else might get invited to fill your seat all the time, rather than just when you aren't there. And woe unto you if they start Axis & Allies on a night when you're out, and then don't finish it for 4 weeks.
Friends show up. If they don't do that in SF, that's just one more reason not to move there.
I also note that people in the South flake out or bail more readily than the Midwest. It's really annoying.
It says 'the rate of "flaking" in San Francisco is really high' - the only reasonable reading of that to me is 'really high... compared to other places'. You can only be 'high' compared to something else, and the only thing which qualifies this data point is 'in San Francisco'.