Many incentives are physical in nature. Can't afford food? Drugs and alcohol will make you forget you're hungry. It's more comfortable to sleep on the cold/hard ground if you're so high you can't feel anything at all.

You would be surprised what sort of drugs these people are using. They aren't paying for cocaine. They spend all day begging for pocket change to scrounge up $10 for some crap meth-like drugs who's sole purpose it to make you not feel anything. For the 4-8 hours after they take the drugs, they can forget everything they hate about their life and maybe fall asleep.

Realistically, strangers are quite happy to buy food for homeless people if they ask. So then the incentive is to get money to for "entertainment" and relaxation in form of drug use.