If you're dealing with a university and its veterinary school, then it's no surprise. My previous vet before moving wasn't associated with a school, but they also had a mask policy, and once I moved I found another one that didn't care.

There's nothing you can tell these people that will make them care or change their minds. I would just write an email that says until they change their policy to reflect actual scientific data, you and your family and friends will find another clinic for your pets.