This is not completely true. I have long avoided using any textbooks the students have to pay to access. The only exception currently is my intermediate macroeconomics class. I have not found a suitable no-cost supplement that covers all the topics of the class. My motivation is that it's easier to teach a class if you can offload certain topics to a reference, and you can't do that unless everyone has access to said reference.

Beyond all that, universities have an incentive to reduce cost as much as possible. Whether university administrators ignore that incentive is left for the reader to determine.

My employer offers small grants to support the creation of open teaching materials: https://lib.k-state.edu/services-support/scholarly-communica...