Well, obviously, but I believe what he's getting at is: if so many students want to take a course, why aren't they allocating more resources toward that course to support demand?

This is all speculation.

Possibly because they treat it as an extra curriculum activity

And as such:

a) Perhaps the lecturers teach in their spare time because they enjoy the activity.

b) It's a fairly specific skill set that would mean hiring new resources for an extra curricular activity that means diverting resources from your primary operation (teaching) which you don't want to do.