Hmm. I wonder if you've ever sat through a chemistry or physics lecture presented by a graduate teaching assistant whose English is frequently unintelligible, mostly because English isn't his or her first language?
Sitting elbow-to-elbow in the equivalent of a basketball arena with 500 of your classmates, straining to understand just enough to take notes, let alone actually learn the material... yeah, students who actually want to learn pay too much money for that level of customer service.
Graduate assistants are stuck teaching these classes because they are a nightmare to teach. The students are often unprepared, unmotivated and even hostile not only to the instructor but to the notion of being held to any reasonable standard of assessment. If colleges raised their standards then you wouldn’t have so many students in the class and you would have tenured professors willing to go back into the classroom in the lower level sections. But now they avoid doing so like the plague and I don’t blame them.