Dr. Doug Knueven say's it best:

What would you think if you went to your doctor, and he talked about the people chow aisle? And he said this is all that you need to eat for the rest of your life, and this is going to be so easy cause you can bypass the fruits, vegetables, and fresh meats. Just go to the people chow aisle, and pick out your favorite flavor. For breakfast take a scoop of people chow and eat it, for lunch take a scoop of people chow and eat it, for the rest of your life. Do you think you could get all the nutrition that you need out of a bag of people chow? Everyone sees how ridiculous that is, and how is it any less ridiculous for pets?