I can’t believe I’m agreeing with you of all people, but I have as well and as strange as it may sound to people who haven’t, there is a certain proportion of the homeless population which is homeless by choice. It’s not a large proportion, but it’s not that small either.
It’s a chicken and egg problem with many schizophrenic and borderline homeless where you can’t tell if they ended up on the street due to mental illness or if living on the street made them mentally ill, but I have personally seen people choose to leave housing projects to go back to the street, and have heard similar stories from others.
That said, of course using this fact as an excuse to not do everything we can to house the majority of homeless who do want homes and to support the ones who don’t in other ways (soup kitchens, clinics, etc and mental health counseling for those who want it) is absurd.
In their defense, it is very counterintuitive. “Who would want to live on the streets?” is a perfectly valid question with an obvious answer (nobody) and a different correct answer (people with specific manifestations of mental illness).
I’m in favor of UBI and I’m in favor of specific investments to solve homelessness, which I think should be one of the most urgent topics on any political agenda worth considering. But you are right when you say that these are two completely different issues.