The biggest reason why there are so many homeless in SF is that SF has the most liberal guidelines for public welfare qualification of any city in America. The city is largely financed by taxes from high-techs, which explains the ridiculously high rents and housing prices, too, as the workers of these companies are making loads of cash. However the extra money is pretty much given away, so all manner of homeless/ homeless-mentally ill move to SF and immediately get on the welfare rolls. Can you blame them? There is no way any city could have this huge transfer-of-wealth without the wealth, and Silicon Valley supplies it. It's that wealth that also drives up the prices of things, contributing to the problems the "underclasses" already have. That's the crazed dynamic at work. Oh yeah, another factor: SF does not enforce laws around marijuana possession or prostitution. Basically it's like hanging a giant "Come here, we love you!" sign for every druggie and streetwalker in the country. Other American cities are, believe me, much better-representative of the typical American situation than SF. SF is, it's fair to say, utterly unique in the American city-scape.