They used to be just normal rentals for people just starting out who didn't have much money. If you watch old movies or movies set in a prior era (like "A walk in the clouds"), this was normal market-rate housing for folks without tons of money. And that's mostly not the case anymore in the US.
Also, the last time I looked -- back when I actually lived in the San Francisco Bay Area well over a decade ago -- SROs in San Francisco were already more than $1000/month.
I would like to read through the several thousand page Catalog of Federal Domestic Assistance, but I'm having trouble finding the time. I haven't gotten very far.
I am trying to create pilot programs and find things that actually work. I would like to stop just shooting down suggestions that don't work and start saying "Well, we did x, y and z in my small town and that got such and such results and here is how you can try to replicate that." But I'm still struggling to make ends meet myself, and yadda. So it's slow going.
In my mind if you have the interest to read HackerNews you would have the skills to get a decent job that would get you off the street. I'm clearly unaware of the obstacles that prevent that and would like to learn more.
I live in South Africa and the inequality here is horrendous but most of that is due to a disparity of education. There is a shortage of skilled workers so in my mind someone with HN interests wouldn't struggle to find enough work to avoid homelessness.
Part of it is that housing policy here has serious issues. We have changed zoning, etc, such that it is nearly impossible to build the kind of walkable communities that existed in this country before the automobile took over. The New Urbanist movement tried to build walkable communities. They found it very challenging to get such plans approved at all.
Another piece is that we don't have good health care policies. Most people on the street in the US have some kind of health issue, whether medical or mental health. Our policies in the US don't do a good job of providing for such people. Most developed countries have much better policies for dealing with people with serious issues of that sort.
Here are a couple of my blogs if you want to try to read through them, though I don't know if that will really answer your question:
https://streetlifesolutions.blogspot.com/
https://sandiegohomelesssurvivalguide.blogspot.com/
I would like to see the affordable housing shortage tackled and the health care issue resolved. I think if those issues are not resolved, basic income will likely actually make things worse for poor people who will then get dismissed as not trying hard enough, not managing their UBI, etc. I think giving UBI to people in the current American climate will just mask the real problems and make people who are comfortably well off even more callous about "Not my problem. It's just a personal issue and the world isn't obligated to fix your personal issue."
I already get told on a regular basis that the housing crisis isn't really a problem. It isn't a problem for the well-heeled, so they don't see why others find it so monstrous and impossible.
SROs used to be pretty commonly available. We tore down up to 80 percent of them over the course of two decades, probably because Baby Boomers were wealthy enough to not need something that cheap. We never rebuilt them.
I think rebuilding them would be a step in the right direction.