One could view this as a by-product of the research university vs the teaching university.

A research university is supposed to be a place where experts at the bleeding-edge of their fields congregate. That expertise is expected to trickle down to their undergraduate teaching, and so the argument goes that only they are qualified enough to prepare those courses. I think this is all largely unwritten and largely unsaid, but undergraduates would be pretty unimpressed if they were getting standardized lectures.

A teaching university isn't bound by that sort of expectation, so one could imagine that courses can be more homogenous without penalty. Lots of textbooks provide "sample" slides to use, and if you're teaching out the textbook anyway, you may as well use the slides you're given. Having had no experience at a teaching university, I can't make any claim as to whether this happens with any regularity.

Good question. The short and unsatisfactory answer is because "that's the way it's done", and any professor who decided to copy someone else's course would face heavy criticism, and neither the students nor the department would feel like they'd be getting their money's worth.

The system could work a different way, but it doesn't, and you can't really get there from here.

You can change it to "courses must be prepared for" and it's still true. Even teaching a course that you know the material for backwards and forwards takes time to prep for - if you want to teach it well.

Many universities do their own course prep for 2 separate reasons.

1. Schools recruit by bragging about famous/accomplished graduates -- all taught by the $UNIVERSITY_NAME method.

2. Many schools research teaching methods, and therefore new things and materials are actually part of some experiment.

There is actually a 3rd, sort of meta-reason. In academics it is considered "incestuous" for students to undergrad->phd->position at the same university. They like to send people around various places as a way of spreading ideas. This mostly plays out in graduate levels, but that same thinking applies to "different schools teach different ways".

A final thought -- just like it is good to have multiple varieties of plants in fields, multiple brands of networking gear in a security conscious network, and so on, perhaps multiple teaching methods are good too, as a way of innocculating against "bad thinking".

When I started the podcasting program at Tulane's med school after Katrina, we were at Baylor. I was in the audio booth when one of the techs realized he hadn't recorded the Baylor prof's lecture. He ran up front and asked him what to do. His direction: "just post last year's recording."

That said, my mom teaches math a community college and she teaches in a very particular style (good German upbringing). Her first semester teaching any particular course involves a lot of late nights.