My kids went to a western-based university.  About 2/3 of the classes were taught by sessional instructors.  These are contract workers with low pay and no job security.  And yet the teacher ratings (also available at Ratemyprofessors.com) revealed that the sessional instructors typically scored noticeably higher than the full-time PhD profs.

Why?  Because tenure and promotions are based on research and publications, not teaching.  Teaching is often seen as an unwelcome distraction from the real work of research and publishing.