Before you downvote me, please, go hang out in Pioneer Square in the middle of the day.
Addiction is not, per se, a major root cause of homelessness. There are people with addictions who have careers. If addiction caused homelessness, then getting addicted would land people on the street as a routine consequence.
People see homeless individuals do X and conclude "X causes homelessness" or is otherwise some inherent trait of "those people" and it's usually unfounded.
People on the street are often using street drugs in place of prescription medication, sometimes by preference because, for example, they have a psych diagnosis and street drugs have preferable side effects to the medication they are supposed to be on.
I knew a bipolar man somewhat well who was homeless. He smoked marijuana in preference to the prescription drugs he refused to take. He was unemployable due to his undamaged mental health issue, not because he toked. Toking was just how he got by. He wouldn't have been more functional without pot. He likely would have been less functional.
We still don't have a slam dunk cure for most mental health issues. As long as this remains true, it is problematic to act like addiction is not merely a side effect of the problem and, instead, to pretend it somehow causes the problems an individual has.
Saying addiction is the problem is a little like blaming chemotherapy for ruining someone's life and not acknowledging the underlying cancer as a problem.
The National Coalition on Homelessness seems to agree, citing "A 2008 survey by the United States Conference of Mayors asked 25 cities for their top three causes of homelessness. Substance abuse was the single largest cause of homelessness for single adults (reported by 68% of cities). Substance abuse was also mentioned by 12% of cities as one of the top three causes of homelessness for families. According to Didenko and Pankratz (2007), two-thirds of homeless people report that drugs and/or alcohol were a major reason for their becoming homeless."
The American Public Health Association as well states, "On an individual level, persons who have a problem with alcohol and/or other drugs, and who are in marginal economic circumstances, are at especially high risk for homelessness."[1]
The road to addiction is complex, and yes mental health treatment should absolutely be better in the United States, but to claim that addiction is not a risk factor to homelessness is disingenuous.
[0] http://www.nationalhomeless.org/factsheets/addiction.pdf [1] https://www.apha.org/policies-and-advocacy/public-health-pol...
What I'm trying to tell you is people blame drug use when it is really more like a symptom than a cause. It's like saying "The pain killers did it."
The conclusion winds up being that if people wouldn't do drugs, their lives would work. The reality is more like if their lives worked, they wouldn't do drugs.
Again, it's like saying "Chemotherapy causes homelessness!" because some folks on the street are having a medical crisis. If you just stopped chemo cold, it wouldn't get them off the street and now their cancer is going unchecked.
There are things you can do that help, like improve the crappy American health care system. But "Just don't do chemo" isn't an answer but that's exactly how substance abuse gets addressed.
Think of it this way: Most people on the street are male. No one goes around saying in all seriousness "Just don't be male. Problem solved!"
First, you are going to a known location which increases the concentration of one type of homeless person in space.
Second, you are looking at a particular point in time, which will increase your odds of seeing chronic homeless people versus people who have had a brush with homelessness.
Mental illness and drug addiction are absolutely a big part of the chronic homeless problem. Either as a cause (schizophrenic people are more likely to wind up homeless) or as a negative feedback loop (extended extreme stress can made addictive escapes more tempting, and can cause a psychotic break).
However drug use and mental illness are not the main things determining whether temporary homelessness becomes chronic homelessness. And programs that attempt to solve that should look very different than ones which try to help the chronic homeless.