Something the other replies didn't mention is that some (many?) homeless people wouldn't be hired for minimum wage jobs because they'd be considered overqualified. [0]
There's always the option to leave education and employment off of your resume, but during the interview how do you explain even a four-year gap without mentioning "I was doing a Ph.D in particle physics" or "I designed marketing materials for Goodyear"?
To expand on this: low-end employers want people who (it appears) don't have any better options. If someone seems like they could find a better job at some point in the future, then they're less likely to put up with the kind of treatment that minimum-wage earners usually get. They might quit, or they might cause trouble and have to be fired. Better to go with someone who knows that they can't do any better.
That's been my experience. I had a white-collar job, quite well-paid, which I lost due to layoffs and the business moving to a cheaper part of the country. There's a huge industry downturn just now, which forced me to look into retail jobs. I did, and it was really, really difficult to get one because the people interviewing me knew that I'm only there until I can get back to what I was doing before.
This reply is interesting. I wonder if this doesn't apply at other levels as well. For example, a small business with a tiny engineering team not wanting to hire an engineer qualified to work for a top tier startup or tech titan.
There's always the option to leave education and employment off of your resume, but during the interview how do you explain even a four-year gap without mentioning "I was doing a Ph.D in particle physics" or "I designed marketing materials for Goodyear"?
[0] For example, a "Emmy-award-winning, New York Times bestselling author and Harvard grad cannot land a job as a greeter at The Container Store." http://www.scarymommy.com/club-mid/the-illusion-of-control-o...