With prescriptions, it is not about what the customer wants, it is about what brands the insurance wants to cover (and getting a doctor that does not write a brand specific prescription). If an insurance company only covers a weird brand of a common (but expensive) medicine, the customer either has to hunt for a pharmacy that has it in stock, wait for their local pharmacy to order it (in either case delaying when the insurance company has to pay for it), or buy the in-stock brand without any insurance coverage. The insurance can still claim they cover the drug while paying less for it.

At one point, I was on a medicine that had a very high co-pay for the brand name and would not cover the generic. It was so high that it was cheaper for me to buy the generic uninsured instead of paying the co-pay.