Teaching standard isn't always the focus of universities when hiring staff. Better researchers help them up the league tables, better teachers do not.
There's nothing to say that someone couldn't do both, but masters of both teaching and research are quite difficult to find compared to someone who is passionate about one or the other; generally a teacher likes to teach and a researcher wants to work, ergo teaching is something of a distraction and in addition, could take them out of their comfort zone. All the **** teachers I've had have been brilliant academics.
It's usually the case that, in order to get the best grades, you need to do your own reading and research. This is true whether you're being taught by a stuttering slide-reader or a dynamic natural speaker. Lectures are the introduction to the topic. University is more about taking responsibility for your learning than having it fed to you, lecturers should introduce the topic and explain what you need as a guide, but the learning would come either from your own reading of the sources/reading material they advise, any other material you find yourself, reading journals, or from lab work, projects and researching for essays and exams.