We used to have institutions for the batshit fucking insane. Then came deinstitutionalization and we closed them in favor of the supposedly more humane community-based care organizations. Look how good a job those organizations are doing.

But why have I never seen this in Western Europe?

We have mental people here too. (And homeless mental people, it's not like all of Europe is some social democratic paradise.)

I think the root cause in SF is the crisis of homelessness which seems massive compared to what I have seen elsewhere.

Exactly. Apart from that, we have fecal collection bag dispensers (bin + bags) for collecting dogs' feces. It is emptied on a daily basis. Most homeless people here actually use it, apart from going to a homeless shelter which is another option.

SF has a weird advocacy culture that keeps people on the street. I can’t recall any other US city where this problem exists at scale.