> but paying for a drug habit when drugs are expensive and you have a hard time holding down a 9-5 job (usually due to mental illness)

I would imagine mental illness would be a bigger barrier to handling a 9-5 job than drug use.

IIUC, the majority of people that drink alcohol and smoke weed daily work 9-5 just fine.

Is the same true for the majority of people that have mental illness? What are we even considering mental illness?

I don't think the visibly homeless crazy people have mild mental illness. I think their mental illness is a bigger problem then drug addiction if they even have that problem.