There are very few positions I've ever had, even great jobs I loved, that I wouldn't readily leave for greener pastures. Why would I stay when another company is offering me more money and/or more responsibility for similar or more interesting work? Sure, I'm not going to drop my job at the drop of a hat, but I'll do what's best for me and that rarely involves staying for long periods of time at one company.
I've had a boss that had no choice (it was an Austrian firm that refused to pay US market rates), but to hire people grossly overqualified for the position they were in and paid way below market rate for it and he put together a great team by finding "naive and/or desperate people" (I was both at the time, having just come back from Japan when Nova, the English school, when bankrupt suddenly). And I'm still grateful to him for everything I learned there about business, but he did his best to keep everyone advancing in their career and encouraged people to move along when it was time.
Just as companies keep trying to make employees interchangeable that makes jobs just as interchangeable.
John Smith the high school dropout is glad to just have the job, he'll stick with it as long as he can. The emmy-winning harvard graduate is going to be looking for alternatives every day she's working there, because she knows she can get hired at one. The likelihood of them leaving is much higher because they're coming to those opportunities instead of the other way around.
Working retail is simple (not easy, depending on your personality) and anyone can do it once they've been shown how to run the register.
"The likelihood of them leaving is much higher..." Do you know what the turnover is in low skill jobs?
http://www.cnbc.com/id/102021496#. "After reaching 100 percent in 2005—then subsequently dropping to 51 percent following the recession in 2011—median turnover rates for part-time retail workers jumped back up to 74.9 percent in 2013, according to Hay Group, a management consulting firm."
Even call centers (considered a decent job for "unskilled" adults), have better than 25% turnover. http://smallbusiness.chron.com/standard-employee-turnover-ca...
I've had plenty of jobs that were beneath me and I hated (I even worked at Taco Bell for a season as a graduate student), I did them and did them well then left when I felt it was time to move on, no harm no foul.
And this includes computer science and programming jobs. Yes I enjoy programming and engineering and generally like my job, but all this fake enthusiasm is annoying. Most people are there for one reason to make money, either now or in the future.
The statistical improbability of getting hired for such a position aside, if someone overqualified wanted to work for you in a (very) temporary position, why would you not hire them? Or at least get them to interview?
Life's complicated enough without trying to overlay my preconceived notions about why they're appling for a particular job onto someone else, as you say.