I spent four years studying physics and mathematics, and I only had one professor that used transparent acetate sheets with notes pre-written and examples on them. All my other professors wrote out everything with chalk. Heck, we were excited when they used more than one colour of chalk, because that meant there was something exciting about to come up.

Additionally, having chalk & talk meant that many students showed up to class, instead of just relying on "reading" printouts of slides that the professor made available. I would venture a guess that I would have performed rather poorly in a class that had powerpoint slides, due to my inherent laziness in my early 20s.