It's not. Young people with no money move here (like to any other city) from Podunk and make their way up the ladder. If they're in tech, the end point is much higher and you get there faster.
You're not going to see a Tenderloin drug addict or one of the Mexican day laborers waiting outside Home Depot make their fortune here. Nor will you see someone like Kevin Rose reduced to working at Burger King, however much value he destroys.
That's true, but that's the case pretty much everywhere. The difference here is that the Mexican day labourer's son/daughter can learn to code at school and completely change their fortune by the time they are 22.
That's pretty significant. In a small town in Minnesota, people won't follow that path because it would be completely outside their experience. Not so here.
I wouldn't say it's that small a value. If there was no relevance to where you started in the life, the odds of reaching the top 5th from the bottom 5th would be 20%. 12+% is where San Jose and San Francisco stack up, which seems pretty spectacular considering all the potential factors pushing against someone.