Because quite simply you are not taking the benefits from complete protein sources.
Why take carnitine (lysine and methionine) for weight loss if you are eating a protein rich diet?
Why take arginine for heart health if you are eating a protein rich diet?
Why take glutamine for intestinal repair and ulcer relief if you are eating a protein rich diet?
Why take lysine for cold sores if you are eating a protein rich diet?
Why take tryptophan to help you sleep if you are eating a protein rich diet?
Should I go on?