For the same reason a medical doctor won’t, it’s both unethical and illegal. Without physically examining an animal, it’s impossible for the veterinarian to come up with an accurate diagnosis and rational plan of treatment.
A veterinarian can’t make a diagnosis based on symptoms only as observed by an owner. The outward signs may be an indication of any number of internal causes with a wide variety of clinical treatments. A complete physical examination and possibly other diagnostic tests are required to determine the cause of the symptoms and best course of treatment. And some medications are considered prescription medicines and it is illegal to dispense such items without a physical consultation.