Don't know about the homeless in the US, could be more driven by poverty I guess. Here it's largely a matter of mental illness, and not the sort that's readily overcome with therapy.
There was a homeless guy that used to come into my work pretty regularly (mostly he was just after illegal wifi boosters for his laptop, along with other components for easy portable wifi snooping). Apparently he used to work in IT, one of my colleagues knew him, but his level of social paranoia made even basic interactions with him nearly impossible.