A large part of supply and demand is the amount of immediate liquidity. Employers seem to have the upper hand when hiring typically because they have a bunch of options right then whereas few applicants have multiple simultaneous offers and if they do, they don't have many. But that's not the same as supply and demand of labor on a longer timescale.

Somebody who's thought more and more clearly about this could write something better about it, but I felt like pointing out what I think is overlooked.