Because they often have friends and family in the city. It is inhumane to force people to drop their entire social networks simply because they are too poor.
I think I might have phrased it as "it's impractical to force people to drop their social safety nets", or maybe "it's unrealistic to force..."
People need help. Not all the time, but at least sometimes for sure. If you can't afford to buy help then you either suffer or else lean on someone else. Asking people (who can't afford assistance) to move away from their friends and family isn't a realistic ask.
It's like they said in that episode of Firefly - "When you can't run, you crawl, and when you can't crawl - when you can't do that... You find someone to carry you."