I went to a relatively large public school with a big endowment. The "just teachers" were called Adjunct Professors and they really had a shit deal. They basically didn't make any more money teaching at my school than they would teaching at a community college and I don't even think they were considered full-time with benefits. Meanwhile, my research PI was a tenured professor who made a very decent living (salaries of teachers at the school were publicly available) but almost all of his salary was paid out by research grants, not the school.

Really among most of the department, I was surprised how much of an afterthought teaching seemed to be. They were by no means bad teachers, and I guess when you've been teaching the same class for many years it becomes pretty effortless, but there really is no good incentive for them to sign up and teach a bunch of extra classes when research is what they are interested in and pays for their salary (along with the salaries of all of their lab members).