It goes deeper than that. Analysis shows a lot of classes are being taught by assistants, while researchers are guzzling money. Academics is getting short changed, while researchers (many churning out liberal studies to support progressive legislation and regulation) are feeding like a pigs.
If you are a professor, you are expected to publish not teach. I was fortunate to go to Iowa State at a time when the Engineering dean didn't think so, but that was not common.
Most of the non engineering classes I had were taught by foreign grad students. In fact seeing an American grad student was so rare it was commented on. The reason? They are cheaper.