It's more nuanced than that. People who are homeless sometimes have preexisting conditions that caused them to become homeless (mental issues, substance abuse issues) and obviously their addictions just get worse.

People who are on the street turn to any kind of drug they can find to numb the harsh realities of living on the street. Alcohol and cheap opioids.

It would be a struggle to find a homeless person without some kind of substance abuse problem. Addict to homeless process, or homeless to addict process.