Professors not knowing how to teach is common for research universities in the US. Their main job is research, and teaching is just something they do on the side.
I'm a software engineer. I also coach girls basketball at the youth level in my after-work time. I read about teaching methods, how people learn, and I talk with teachers whenever I can. Often they can help me get difficult lessons across to the girls.
It's fine for professors to be great researchers. But, if they have to teach as part of it, they should strive to be good at it.