I can answer this, I lived there since 2005 until this month.
The problem is once you stop enforcing the law it creates incentives for all kinds of people to try it out. This created a diverse population of people who are living outside, from mentally ill, to seemingly normal young people fixing bikes in their tents, to dangerous drug addicts that won't hesitate to stab you.
The problem is there is a law and it is not being enforced. People take advantage of this. Thinking everybody is a poor soul that would buy housing if it existed is an extremely naive view. The homeless population is very diverse in terms of reasons they are out there. SF leadership treats all of them as if it had one solution, so of course that will not work. Some people out there want to be there. You can see they are young and don't mind sleeping in a tent. Others are just out of their minds and need intervention ASAP.
RIP SF