I have my own experiences interacting with the homeless in seattle. They're a bunch of overlapping groups.
Around 1/4 are short term homeless, just folks at the margins of society who fell off the page, and now cannot make ends meet. Alot of these folks do find their way off the streets of their own, but this is the group that we need to most urgently help.
Around 1/2 are long term chronic homeless, often they were just short term homeless folks, but now have fallen into the trap of the homelessness. They want out, but cant figure out how to navigate their way there.
Around 1/4 are willingly homeless, as in they do not want help.
And of those who are homeless, they seem to have the same common problems:
Around 1/3rd do have some measure of substance abuse issues (more prevalent in long term chronic homeless).
Around 1/3rd have moderate issues with excessive alcohol consumption (more prevalent in long term chronic homeless).
Another 1/2 have untreated mental health issues of some sort (seen in all groups).
The most common reasons the interventions for homelessness seem to fail:
1. Requires giving up too much personal freedom/dignity - can't come and go as you please/its treated as a pseudo-probation, have to go listen to a preacher, cant bring your pet (usually a dog).
2. too many prerequisites/hurdles to get help (must be sober/clean, must have kids/not have kids.
3. shelters feel less safe than being on the street (your stuff gets stolen, you'll get gay bashed).
4. requires you to give up too much of your SSI or other benefits to get it.
5. long waiting periods for help or not enough bed slots.
It's why I believe in a housing-first approach, housing without hurdles or prerequisites, its much easier to provide services to a sedentary population than it is a mobile one with no fixed address.
I think the case that every homeless person is a drug addict is wildly overstated, but I think the solution that SF and Seattle is doing wildly ineffective in actually solving homelessness, if anything, it makes the problem worse, while also reducing quality of life for folks who do have homes and live there.