Sure, it may not be the leading cause. But the homeless that people are usually referring to when they talk about San Francisco's homeless problem, are the ones with mental issues or addiction problems. I rarely ever see the regular, sane, homeless people sprawled across the sidewalk in the middle of the day. It's always the drug addicts or the mentally ill.
Well sure. People come to the conclusion that most homeless are crazy or addicts based on what they see. Of course they only ever see crazies and addicts. They're the loudest, the weirdest, the scariest, the least self-aware, and the least likely to have any kind of refuge.
The rest of the homeless are hiding from the myriad threads, difficulties and humiliations they're exposed to on a daily basis. The regular, sane, homeless people don't want to be sprawled across a sidewalk in the middle of the day. They want to watch over their stuff, or try to find food or medical assistance or clothing, or solicit donations, or try to contact estranged family/friends, or whatever else they can do to survive.