Economically speaking, it's vastly cheaper to keep people housed through rental assistance, rent control and other measures than trying to re-bootstrap someone into housing. Direct assistance (which most western governments have already discovered) is the most effective way to keep people from becoming homeless in the first place.
Without strong renter protections federally, states are left to implement measure that prevent homelessness, results vary. Even city by city, a renter's rights can vary wildly.
SF is a unique case, specifically because of the political corruption and absolute money printer that the homeless "problem" creates. SF spends millions of dollars in "aid" which means it mostly ends up getting passed through 4 or 5 non profits and only 1/1000 of the money raised via taxes is actually getting to the people who need it.
My best guess is that with the legalization of weed, dealers who used to sell it were pushed to sell harder drugs. These harder drugs come with more significant effects in destabilizing mental health.
Only my opinion, but it’s all about the opiates. The few European cities that I know of that have had a similar scale homeless situation also had a similar situation re: healthcare and pharmaceutical industry - they’re all in Switzerland.
The Swiss “solved” the issue by cracking down on pain killer issuance (like we did in the US) but also by sealing their borders off to drug smuggling to a degree that is impossible in the US. Zurich ~10 years ago looked a lot more like San Francisco than you’d think.
As long as the scary part of any city is where you go to buy, use, and not be arrested for using, that’s where the scary part of the city will be - it’s a self sustaining sadness loop. As far as I know there is no fix besides removing access to the drugs, and for better or worse, that’s not possible in the US. The only “solution” is police crackdowns, which doesn’t cure addiction.
The real “freedom is slavery” argument isn’t too many choices in the grocery store: it’s the ability to choose heroin a couple times when you’re sad and stupid and young.
Obviously it is policies and San Francisco has stuck to the policies against all evidence to them not working. Moreover, the politicians that put in place those policies get reelected and elected to higher offices. The governor of California is the former mayor of San Francisco under whom the situation has turned "dire," as many have attested.
I'm especially thinking about the difference between European cities and US ones where the difference in scale of the issue is notable.