Before I had children, I had no idea how difficult it was going to be, even putting aside finances. Most people without children just don't get how much more difficult ordinary life becomes. There is a deep reason that the "boring" suburbs beckon.
Yes, the schools and the lower housing costs are a part of it. But there's also just the easier lifestyle, where you don't need to compete as much for everything. Parking, sidewalks, aggressive cars, daycare, schools, a seat on the bus, and very much avoiding crazy people who might harm your children.
It is extremely stressful to walk your young child across a busy intersection in SOMA where a driver is aggressively trying to turn, where a crazy person is screaming at the top of his lungs, where a street that smells of urine awaits you on the other side.
Putting SF aside, it's extremely difficult to maintain a good relationship with your spouse when a demanding job takes up a huge amount of time, and when that job seems like a vacation compared with the weekends with the kids. Almost universally every parent I know says that going to work on Monday is a relief, the beginning of their temporary vacation from the intensity of nuclear family life.
The children themselves create the vast majority of the stress. Parents need environments that help reduce that burden, not merely keep inconveniences at bay. At our school the #1 wish of parents, by far, is for faraway grandparents to move nearby and help. If they could afford it. If they could survive in this city hostile to the elderly. If they were willing to give up their comfortable retirement travel adventures to shoulder some of the burden. Very, very few have this arrangement in SF.