The problem with the "but they have mental illness to deal with" answer is that while it may be true, it's very much the edge case, and not the rule.

I know this because I've been homeless. I'd say greater than 90% of the folks that I encountered in my homeless period were there because they were content to be there.

If they wanted to change their lot, there was more than ample opportunity to do so, between available services and working nearly ANY kind of job.

It took me about 4 months of working to get off the street.