I think a lot of the time vets receive their training on dog nutrition from (get this) dog food companies. And of course each dog food company is going to say they have the best food and blah blah blah....
Or it could just be the food they sell there in their office. If you start to feed your dog their food because of the recommendation, maybe you'll buy it right from their office. The more food they sell the more money they make.
Personally, I would never even consider feeding my dog Science Diet, Iams, Eukanuba, Purina (I could keep going) because of the grain contents in the food.... In fact, my pup doesn't eat kibble at all. She gets a raw diet.
I'm not sure what kind of SD the vet gave you, but their website lists the ingredients for the
Small Bites Puppy The first ingredient is CORN the next is a meat BY-PRODUCT. In fact, I don't see any whole meat sources in the food at all!
If you compare the SD ingredients to
Chicken Soup, IMO, Chicken Soup is WAY better.
I'm sure there are plenty of dogs that have eaten Science Diet (or Iams or Eukanuba or Purina) their whole lives without a single problem... but no one food is perfect for every dog. If Chicken Soup works for your pup, then by all means keep feeding him Chicken Soup!