Between 2006 and 2008 I spent a lot of time on the streets interviewing homeless people for a documentary film I was making [1]. One of the things they often mentioned was the "homeless-industrial complex" (a phrase coined by one of my subjects, not me). There are a lot of organizations that make a lot of money "helping" homeless people, and their continued existence depends on there not being an effective solution to the problem. Homeless people are literally their bread-and-butter, so they have a perverse incentive to ensure a steady supply of "product". Similar perverse incentives play themselves out in other areas of our society, like privately run prisons.

[1] http://graceofgodmovie.com