As a new Faculty member teaching Mathematics at a large University, I can see exactly what you are saying happening around me. The main issue comes down to time commitment. Part of my teaching is from a (department mandated) publisher-supported system. This does force the students to pay around $100-$200 for course materials, but it is a huge time saver to the instructor. I also have a more open course, using only open source materials, including an online homework system. This, however, takes a lot of initial, and continued, time investment from the instructor. I believe many people would find the latter preferable, but it seems to be a minority among departments.

There are many alternatives to the system we are heading towards; there are just not many people willing (or able) to put out the time and effort to do so.