It's much easier to just lump all homeless people as schizophrenic heroin addicts then normal people who fell on hard times. Then they are nothing like you, and there's nothing to do about it. "It's just how some people are", but not you or anyone you know. If homeless people are like you, but had some bad luck, that's terrifying. It would also mean that they could be helped, and then society would have to do something/change, and that's clearly unacceptable.