Under highways, particularly, also have the political advantage of reduced visibility.
As a bonus (for the people who don’t want the homeless around), these locations (near and under highways) are also extremely unhealthy, particularly for people with respiratory conditions (and staying in them is a good way to develop such conditions in the first place), so it helps reduce the population served.
The liability alone that Wheeler's proposing is incredible. It'll cost tens of millions to set up (by his estimate) and there'll be lawsuits against the city within days, guaranteed. Nobody will be helped, hundreds will be harmed, and people who aren't chronically homeless will wind up in jail. The camps won't have enough capacity, so nothing fundamentally changes on the ground. Nobody wins.