This is really important to emphasize.
I've never been homeless, but I did spend two years hanging out with homeless people while making a documentary film. The range of people on the street is almost as broad as the range of people off it. I met homeless people with Ph.D.'s. I met homeless people with florid schizophrenia. I met homeless people with substance abuse issues. I met homeless people who were perfectly ordinary but had some bad luck (ranging from losing their job to losing their entire family in a car crash). I even met homeless people who chose to live on the street (or in their car) because they wanted a nomadic lifestyle.
But no matter how you slice it, the only real solution to homelessness is housing.
If the point of the lives we lead is some seemingly positive direction for the human soul, individually or communally, maybe the single cause of homelessness is that we have lost our way, perhaps irrevocably.
But maybe we can be optimistic and imagine things becoming so horrific in the future that the survivors stop doing the things that led us to build a world that either wears down the people or the living ecosystem or both.
It's hard to walk past a homeless person without condemning the society you live in. It's even harder to do anything substantial to change their situation, especially yourself. But to not blame anyone is just morally disorienting. Eric Weinstein, in a podcast that was posted here a day ago, kept harping on "there are intentional causes and emergent causes."
But maybe people are just basically hypocritical lying jerks.
(Do I need a sarcasm tag here?)
That still leaves them wearing the same filthy clothes. It doesn't remotely solve the problem. Furthermore, homeless showers tend to be as horrible as most homeless services.
Either you have no privacy -- because why the hell should your dignity matter when you are homeless? -- or it has mold issues, etc.
All services specifically designed to service the homeless population tend to be really, truly terrible in a "you would get sued if this were aimed at middle class people" kind of way. The general attitude is "beggars can't be choosers" and "don't be so uppity -- show a little gratitude for the shitty service you are getting for free, you ungrateful whiner."
So, no, that's not the solution. Missing Middle Housing is The Solution here. Or one part of it.
There is no single cause of homelessness. There is no single solution.