The simplest hypothesis is that drug use and mental health lead to homelessness. And if that's the case, then the problem isn't housing costs (though high housing costs are a problem, just a separate one), it's that people have addiction and mental health issues.

The problem with your simple hypothesis is that the data doesn't back it up. Drugs & alcohol account for 15% of the reasons for homelessness in SF, and medical issues/illness account for 7%[1]. The data shows that housing costs and financial instability are the largest causes of homelessness in SF.

[1] http://hsh.sfgov.org/wp-content/uploads/2017/06/2017SanFranc...

Sorry, I should have been more specific: I am referring to visible homelessness, people living on the street, etc. The colloquial meaning of the term 'homeless', in other words. I should have been more specific, especially given that this confusion regarding the use of this term is a pet peeve of mine.

No problem, thanks for the clarification. It's a pet peeve of mine, as well.