Because it appears that colleges and universities have taken economic theory and applied it. Colleges are businesses selling to student consumers. The colleges are often making a good profit, which of course is used to build buildings, to hire assistant deans and other administrators, while hiring fewer full time professors and more adjuncts. I can't help but wonder if the consumers are getting their monies worth, or simply a degree along with debt.
There's some interesting statistics about adjuncts out there (a lot are being forced to do it for life and to juggle 3-7 courses). If you have a PhD nowadays, you are almost better off becoming a high school teacher in most countries as they get a guaranteed middle class income and a pension. Versus the Adjunct who gets paid per course, has no union nor pension.