It's mostly because the political culture of SF is trying to solve a mental health issue in compromising way, because the real solution set is not a local level thing, but a state and federal level thing. It's same with the current DA, trying to activist create an alternative to arrest vs. using social workers, while such a solution needs more than the power of the DA's office to properly implement. So you get half-assed solutions that don't actually solve anything much.

Also pure corruption. You'll notice there are a bunch of regions where the homeless roam, and bunch where they do not, and there are very good reasons as to why that is the case.