It's anecdotal, but I've definitely had rich friends get high in the open. Even when not homeless, there are a lot of reasons to hide an addiction from the people in your home and instead seek out locations where you can get high.

Agreed.

In my moderately sized southern city of 200k, we logged/censused 576 homeless a few months ago. Of course these individuals were the ones able to be counted; there's likely more. So roughly a quarter of a percent per captia.

As with most US cities of this size and economic demographic we have a few areas where those without housing tend to congregate. Pains me and I wish I could help more than I do. And from what I have seen in my many decades of living here, these areas are totally devoid of this "open air drug market" characteristic. Not to say transactions don't happen they most certainly do, but that's true of every square foot of any US city, and the people in these places shield their behaviors from public eyes as well as anyone else that procures such things. My city is experiencing a staggering increase in violence this past year at a seemingly disproportionately faster rate than other nearby metro areas if my napkin math holds true and yet none of that violence is noted as occurring at these areas I'm describing. I think that's worth noting too.

The interplay of homelessness, mental health, drug dependency, economic struggle and societal rejection is a very complicated and interwoven problem. It's disappointing to hear that there are individuals who feel so self assured in the sources of these problems when it's evident that the qualities of these problems widely differ from place to place.