I wonder if over-qualification in resumes plays a role. In the book Nickel and Dimed[1], the author explicitly hid the fact she had higher education even from colleagues. And whereas she complained a lot about working conditions, finding a job per se wasn't a challenge at all (to the point she could even decide between two options at one point), even being restricted to non-intensive labor jobs due to health reasons.

[1] https://en.wikipedia.org/wiki/Nickel_and_Dimed

Over-qualification definitely makes a difference. An employer will be reluctant to hire someone that's likely to snag a higher-paying job in a few weeks.