What they usually mean is "they can't find an employee in the location they want, with the credentials they want, with the skills they want, at the salary they want, who's been trained by someone else."
This is a highly overconstrained search. If/when employers loosen one or more of these constraints, candidates show right up.
Like everyone, I prefer to get exactly what I want.
...but it doesn't always play out that way.
When I've needed staff and my dream candidate isn't there, I've hired imperfect people that I had to train, raise the rate I was willing to pay, put together attractive relocation packages, etc., and I've never had a problem hiring in 20 years as a manager.
(Both parties in an employment relationship may not have equal bargaining power, but I take issue with the implication that employers have no agency and workers no power.)
EDIT: A point I wanted to include or imply, but couldn't really shoehorn into this inversion, is that just as employers set bounds on what they offer employees, workers set bounds on what offers they accept. Sometimes it's rational to hold out for the right role through an extended period of unemployment (while focusing instead on finding the right role and acquiring or sharpening marketable skills) than to take underemployment that leaves you with no time for improving your position.