So here's the straight poop...
I've lived in Portland since December 2019. Before that I lived in one of the wealthiest zip codes in Kentucky and McMansionVille rural Central IL.
For starters, right click and launch this link from incognito mode. It does a decent job trying to get into the mindset and reasons on why people are homeless.
Asking why we have a homelessness problem is much like asking why American Healthcare is such a shit show. Answer, it's a lot of reasons and it's really complicated. It's not just drugs. It's not just mental health. It's not just housing prices. It's not just employment issues. It's not just people wanting to detach from society. It's all of that. It's none of that. It's a handful of them. And that reason changes wildly from person to person. You can't approach this the American way of saying X causes Y and throw some money at it, see that it doesn't work and throw your hands up and say "well at least we tried". Because that's usually the approach. Each person that is homeless has a very different reason they are there and require very different paths to reenter into society.
For a really bad analogy, think about adopted animals, particularly ones that have been abused. It takes months, if not years for the animal to start trusting you and and letting you pet it. Same thing applies to humans. There have been studies where they have brought people into shelters that are more like apartments with multiple bedrooms in a single unit. Some homeless people instead of staying in the bedrooms will go out and sleep in the hallways because they didn't trust where they were. It took months for them to get comfortable being around others. Now apply this to thousands of homeless. Add in drug addictions, chronic health issues, and general distrust and it's not something you can just scream away.
Specific to Portland. Yes we have problems. They were here before the pandemic. They got worse.
Homelessness specifically escalated once downtown work commuters and general pedestrian traffic slowed to a crawl. At one point downtown traffic was down 85% from previous year. Combine that with cleanup groups not operating because of a global pandemic and it got out of hand. There were several blocks downtown that looked like tent cities. With that comes trash, human waste and drug paraphernalia. There are several places around town that are large encampments. Usually on side street park ' n ride lots where they push in broken down vehicles and just start congregating. There's also places along bike trails where they build up since the trails are easy to navigate and setup in the woods along them.
I ride those trails multiple times a week, my wife does as well. Thousands of other people do every day as well. There's been no significant issues on the trails since I've lived here. I wouldn't want my kids or wife, or hell, even myself riding alone on their at night. But during the day, whatever. There's places with trash, but in a 25 mile ride, there's maybe 300 yards worth of trail that are sketch. That's it.
Around town they do build up under bridges and in some of the city parks. But you have to understand how big this city is and how little actual space these things actually occupy. It's a few streets that are block long in a downtown that's a couple miles long. It's 4-5 tents propped along an interstate wall.
There is a lot of trash. I have seen a takeout tray with human poo in it. I've had to dodge some needles on a sidewalk. Yep, that's something that happens. But welcome to humans. We have core needs of in-taking things to survive, and excreting them. We have vices to blunt the misery. Pretending it doesn't exist doesn't fix it. Telling them to move doesn't fix it. See that article. That article is a great analogy. It spends pages going over the problem, you find out the antagonist is evicted, only to pack up his stuff and move a couple feet from where he was.
Humans are programmed to survive. We also are programmed to push boundaries. That's how kids learn. Combine survival and boundary pushing and you see some of the ingenuity you do with homelessness.
My wife works with heart failure patients. Roughly 80% of her patient base are either currently on meth, or have used it recently. A huge number of that 80% have also had issues getting or staying in a permanent housing situation. Now add in medical bills and you see how this cycle starts.
America has a lot of issues. Very few people want to actually talk about them in good faith.