Because, it is controversial as to whether or not probiotics and such do that much, if anything.
And sometimes supplements can cause harm. Example:
http://skeptvet.com/Blog/?s=probiotics
In response to: “I’ve starting give her a probiotic and am going to switch shampoos to something to treat subhorreic dermatitis. (Both through my own research, which to vet said would be good. Just wondering why they didn’t suggest it themselves in the first place.) Maybe it’s time to switch vets again”.
PS: Some of those shampoos can be very drying and irritating to the skin. My dog with allergies gets bathed once or twice a week with Malaseb in conjunction with other treatments.
