Why are people so afraid of hobos?

I saw my old friend from school on the side walk, as a hobo. He looked way skinnier than I remember. I had stopped and given him some dog food for his dog and water. He didnt seem to recognize me and said "god bless you" like he would to a stranger. Think of yourself driving down a road and you see an old friend being a hobo. Wouldn't you want to stop and help? I imagine that every homeless person has someone they know that has seen them and won't do anything for them out of the fear that they're hobos and scary or dangerous. Why are people so afraid of hobos?