Because solutions that work (Free mental health care, free housing, more social workers) are not palatable to the people who live there, despite the city having more than enough money to actually pay for those solutions.
It's hard to imagine surrounding cities are different because of free mental health care, free housing, and more social service support for homelessness.
Homelessness is not a problem that affects all cities alike.
It's not the case that Daly City, Mountain View, Pleasanton etc, have all solved the problem without telling SF how to do it. In reality, the surrounding cities simply don't have the problem for various reasons and environmental factors, so they don't need to solve the problem in the first place.
My experience in very poor metropolises across the plamet is that it's an entirely simple problem to solve. My experience in some of the most expensive cities on Earth is that providing housing for the poor devalues the only nestegg most people have and is widely railed against by the masses.
Its a Ponzi scheme first and foremost, housing simply cannot outpace wages, it's impossible.
When it all comes crashing down is someone elses problem though hey :)
Versus the outlying suburbia? Because it's harder to get around in suburbia. Cities are easier to walk / ride your bike around and you're just a face in the crowd. People notice you if you're camped out in a suburban shopping mall.
Or versus another city, it's the weather. Never hot enough to turn you into jerky, never cold enough to turn you into a popsicle.
Since piles of people dead from exposure isn't a risk, the city doesn't have the same motivation to build shelters like a New York does, so they kick it to the side.
The citizenry gets concerned with poop on the street, but once it's bodies, they really expect the government to do something.
So, now that the government can safely do nothing, why won't the private sector fill in?
3 part answer:
1) my understanding of the political lean of the city is that the government is the great social savior.
2) most private groups who take care of the homeless and otherwise destitute havehistorically had a religious affiliation.
3) private groups have to deal with the same real estate prices as everyone else, and because of 1 and 2 and the general skepticism of religion by the citizenry, they won't be given any tax breaks or land price cuts, meaning they simply cannot afford to setup service.
It really is a self feeding system of misery-but-not-enough-to-kill-you all the way around.
There is literally no money in it, so obviously the private sector isn't going to do anything. You might get a bit of "third sector" charity, but it's always inadequate wherever you go.
In my city if I were homeless I'd want to be central not in the suburbs. There are better services for homeless, more other homeless people to blend in with, opportunities to beg, places to find shelter etc.
As of July 2019, there is no know way that cures drug addition with even moderate degrees of success.
Indeed, even the very definition of addiction is still
controversial. From [1]: "[D]efining addiction is not an easy task
and still represents a considerable source of scholarly dispute. The
term addiction has been and it is still used in at least three
different ways. [(i) I]t is used as a lay term, which entered the
English language in the late sixteenth century (maybe owing to
Shakespeare) to indicate inclination or proclivity for certain habits
or activities, in both its positive and negative connotations,
including excessive drinking and smoking. [(ii)]Since the late
nineteenth century, addiction is also used as a medical term to
indicate pathological, compulsive drug use [...] [(iii) A]ddiction is
used as a psychological construct to indicate a compulsive
motivational drive [...] Instances of all three meanings abound in
the scholarly literature, sometimes within the same paper."
OTOH, drug dealers, too, obtain economies of scale by concentrating drug users in close physical proximity (and vice versa). It seems to be plausible to assume that the easy availability of drugs is itself causally contributing to drug use and drug addiction.
[1] A. Badiani, Is a 'general' theory of addiction possible? A
commentary on: a multistep general theory of transition to addiction. https://www.ncbi.nlm.nih.gov/pubmed/24888430
So an endless parade of soup kitchens it is.