The true, and sad, reality is that a majority (up t 70%, in some institutions) of the teaching is done by sessional staff on contract, for a fraction of the rate paid to full-time profs, and with no benefits or job security from term to term.  That said, overall, these sessional instructors are the superior teachers, as evidenced by ratings in Ratemyprofessor.com, for example.

The inconvenient truth here is that the full-time professors don't place their priority on teaching.  The way to promotion and tenure is through research and publication.  Teaching is an unwelcome distraction to this end.  Once tenured (and often, before), they are immune from bad student ratings.  While there are many wonderful exceptions to this norm, it is in fact the norm overall, sadly.

Sessional instructors teach because they want to teach.  It is easy to weed out the bad ones; just don't offer any more contracts.  But they are shamelessly exploited by universities.