The Nation runs an article about universities and colleges staffing many of their classrooms with part-timers, called adjunct faculty or "adjuncts." These often have the same excellent credentials as the full-time, tenured or tenure-track faculty.
We over-produce PhDs. There are more people trained and eager to be professors than there are career vacancies, especially in the humanities and social sciences.
Adjuncts normally don't get benefits and are paid a flat fee to teach each class, something like $4k. Using adjuncts a campus can cover the same classes a full-time assistant prof would teach for perhaps 1/3 the cost, or less.
It isn't uncommon for adjuncts to teach at 2-3 campuses in a metro area and be known as "freeway fliers." They need to live with parents, flatmates, or have a working spouse to scrape a bare living. Many qualify for food stamps.
It is known the number of college-aged Americans will decline year after year. Nobody wants to layoff tenured faculty. Adjuncts are hired (or not) each year as-needed.
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When I began teaching, university business schools sometimes had local attorneys teach business law, and secretarial school faculty teach business writing. In both cases their idea was to pick up some extra $$ in a side gig, and their benefits were normally covered elsewhere.
By the time I retired the system had begun hiring adjuncts as faculty substitutes. I'm sure it is worse now.
I vaguely remember that if we hired an adjunct to teach 3 classes we had to give them benefits (health, sick leave), so to keep costs down the limit then was 2 per term per person. I don't know the current rules.
