In the last decade, universities have shifted teaching responsibilities from full-time, tenure track positions to part-time adjunct positions. Today, half of all teaching positions are part-time and include no benefits. Adjuncts are paid an average of $2,987 per semester-long course[3]. That's less than $1000 a month. If you've ever taught a college class before and realize how much preparation it actually takes, an ostensibly "part-time" position easily demands 40-60 hours of work a week.
In some fields, over-supply is helping drive this trend[4]. We produce more History PhDs than there are jobs to fill. This weakens the bargaining power that academics have on the market. But this does not absolve universities from their role in the immiseration of academics – after all, they continue to accept more PhD candidates with full knowledge that most of them will never find gainful employment in their field. This is not surprising given the fact that departments now rely on _graduate students_ to teach one-fifth of their course loads[5].
The "casualization" of post-secondary teaching is also a disservice to students, many of whom don't realize that the bulk of their instruction is now done by overworked, underpaid adjuncts who don't have the time or incentive to do their best work.
[1] https://www.bls.gov/ooh/education-training-and-library/posts... [2] https://nces.ed.gov/programs/digest/d16/tables/dt16_101.20.a... [3] http://www.chronicle.com/article/Adjunct-Project-Shows-Wide/... [4] https://www.insidehighered.com/news/2010/01/11/grad [5] https://www.aaup.org/issues/contingency/background-facts