The homeless and drug addicts operate outside the law--hence the difference.

Personally, I'd be fine with scooters if they were never used or stored on sidewalks. Which will happen right after bicyclists start obeying traffic laws...

That may all be true (although I certainly was not under the impression that New York or Chicago were particularly clean) and good evidence that with the right policies and programs, those are tractable problems, but I think we can at least agree that nonetheless they are harder to solve than the MTA taking a vote and mailing out some letters about new permitting requirements. So the fact that that has happened for scooters, yet homelessness and substance abuse haven’t been solved, doesn’t strike me as “malevolent”.

This could be an oversimplification, but my perspective having lived in both NYC and Chicago:

New York is dirty. It's very dense for the US and the trash collection is on the sidewalk in front of businesses, meaning you are walking around trash on a daily basis.

Chicago is one of the cleanest cities in the US, largely because of the alley system built after the Great Chicago Fire.

You are fully correct. The reason SF don't address it, IMO, is that it's a no win situation for them. If they clean up the homeless they'll be forced to make tough choices and enforce public order against people their stated ethos which tells them are oppressed and deserving of unlimited compassion. If they do the right thing and clean the place up there will be a backlash from activists. If they do nothing at all then they don't have to make tough choices, don't have to face backlash, and the frog continues to slowly boil which will be the next administrations problem.