45% have mental illness.

25% have "serious" mental illness. https://www.bbrfoundation.org/blog/homelessness-and-mental-i...

38% are alcoholic.

26% abuse drugs. https://www.banyantreatmentcenter.com/2020/07/01/a-look-at-h...

I suspect they tend to be the more visible ones, as opposed to the ones in shelters or assistance programs.

I imagine there are multiple methodologies to measure this because it's hard to even get a handle on how many people are homeless. The point still stands: "they're all just so mentally ill they like to live on the streets" won't wash as an explanation.