Or they're doing the absolute minimum so they can be employed. Quite a few profs I've encountered clearly aren't learning anything in their field any more. They're just being the typical drone, at least as boring an existence as the classes they jam down your throat.

Some I've had are extremely interested & active in their field, and they're all fascinating to talk with, and have useful, efficient classes. I've yet to see someone who isn't interested in their field do a good job teaching it.