re: Why do universities hire professors that students can't understand?

The answer is research and potential to bring in grant money.

Revenue from undergraduate tuition typically gets spent on student services and administration. Thus, teaching isn't really a money-maker for big research universities, which is why they increasingly rely on contingent faculty -- i.e., non-tenure track faculty (i.e., instructors) and non-permanent faculty (i.e., adjuncts).

Grants are where the real money for the big research university -- most take ~50% (or more) of federal and foundation grants right off the top. Most people don't know that. For fields like biomedical research and engineering, this amounts to millions.

Thus, if a professor can bring in research dollars (or has the potential to), they don't really care if that professor can't teach well.