Because they are SO DAMN CHEAP.
That's the reason.
Did you know that teaching a freshman 3 credit course that meets 3 times a week for a semester only pays $3,500 to an Adjunct Professor?
That's 45 hours of class time, plus preparing lectures, grading papers, administrative work, and office hours for students.
If you figure all of that time adds up to 150 hours a semester, that means they are paying an Adjunct Professor $23.33/hour.
This is why their academic ranking keeps dropping. They can't afford many good professors, because they have too many administrators on big bucks. So, they fill out their ranks with very poorly paid people to teach the masses of undergrads.