The evidence disagrees. Housing-first policies in multiple countries have been a smashing successes in reducing addiction, so clearly there are some factors you're missing. For instance, theft and violence is a huge reason homeless people avoid shelters. Having a sanctuary that's somewhat secure from thieves and muggers means they actually add some stability. Stability makes their lives less shitty which reduces the urges to use to escape their shitty situation. That's just one example I've read about.