Perhaps it's due to the difference between CoL and working wages in a given city. Even if someone who is homeless does start working, that doesn't mean they can afford rent in the city. That also doesn't mean they'll get healthcare or other necessary benefits for their continued survival. You also have close to no protections in the US, so you don't really have stable employment in the same way other countries do. So at the end of the day, they may think "whats the point?".
I've also seen a fair number of homeless people who are disabled, mentally or physically. Perhaps that bars them from some jobs. (The ADA disallows barring people from jobs if their disability does not impair them from doing essential work, but I'm unsure about what is considered essential and what isn't.)
These are just my 2 cents. I imagine there's at least some research on the topic.