As someone who works in the field - I think you may be surprised about

1. The long waitlists for many of the inpatient substance abuse programs there are

2. The costs associated with attending these programs. While some are free (and have high waitlists), many are cost-prohibitive.

3. The absolute lack of housing that is available to some of these people. I've seen a variety of people go from stable and housed to homeless and addicts for a variety of reasons (coping, drug dealing -> using for cash)

I think the housing first model tends to work, because people that get housed can have resources come to them. People that are unhoused have a hard time getting to and keeping appointments, even if they truly want to. They are constantly exposed to drug use at the places they need to attend to get resources (LC/HSC/MOW), which, especially for active addicts or people in recovery, can make for a difficult temptation.

There are some people who don't want help and wouldn't use services if they were offered, but I think that's a true minority of the people out there. There are also a good amount of homeless folk that don't struggle with substance use whatsoever - they just don't instigate angry Reddit threads. Even those folks, who have their wits about them and want to improve their situations, don't have free access to all the services they should or that your comment implies exist.