Science Diet likely sponsors a lot of things at Vet school, to get their name out there. Their salespeople push their products at vets. Add to that how little GOOD info there is out there on canine nutrition..and there you go.
I was VERY surprised when I got Scruff and took him for his first check up to hear the vet suggest feeding raw fruit/veg to scruffy, not to rely on processed food for his nutritional needs. Progress is slow but it's coming.