It's a good question and some people just aren't capable of taking help. A friend of mine had an adopted brother (he died as a result of AIDS 15 years ago from prostituting himself for money and sharing needles).
He was young when he was adopted but was born with fetal alcohol syndrome and so that significantly impacted his ability to learn, concentrate, make good decisions, manage money - all through no fault of his own. When you have neurological damage like that, it leads to terrible personal decisions and terrible consequences and you can't really treat it and so it is not even a situation of not wanting help. Many people are on the street with undiagnosed fetal alcohol syndrome. The leading causes of death for people with FAS are suicide and accidents. Can you imagine - leading cause of death being suicide for people born with FAS, which is about 1 per 1,000 based on a quick search (but about 1 in 100 for Indigenous people living off the reserve - maybe higher).
He was the nicest guy, though. But he felt stupid, inadequate, ashamed, alone, and a burden. About 60% are unemployed because they just can't hold down a job and he had several. Many get into trouble in our communities and even if they get housing, they break the rules and get kicked out because they make bad decisions. But that's part of the illness - making bad decisions. Way too many people are still born with FAS. Some need to be institutionalized but there aren't many spaces. What do we do with those who are a major risk? Some can lead productive lives, but still face lifelong struggles, and many are dealt an unimaginably cruel hand at life that can't be overcome. These are some of the people who are on our streets right now - and as noted, many undiagnosed.
) and countless drugged up and violent people all over 102 ave. 
That's sad.
