Why add an ingredient that is not usable to a pet?
Because it's cheap. Why include brewer's rice? Why include corn gluten meal (which by definition is what's left after the nutritious parts have been stripped away)? Because it's cheap and makes the companies more money.
Iams, Hill's, Purina -- they are the junk food of the dog food industry. And yeah, if you were feeding your kid junk food day in and day out, I'd judge.
My ex-husband's labrador retriever survived on Hill's Science Diet. But he had chronic ear infections, his anal glands needed to be manually expressed monthly, and he had diarrhea any time he ate anything other than his kibble (peanut butter, cheese, carrot sticks -- couldn't eat any of it).
But he THRIVED on Wellness CORE. Shinier, softer coat. No ear infections, no need for manual gland expression. Eats cheese, peanut butter, carrot sticks with no problem.
Same for my pit bull. He came home from the shelter (which feeds Hill's) with a rough, scraggly, dull coat. He was itchy. His poops were HUGE. He is now soft, his coat filled in, it's softer. His poops are half the size they once were.