Because some small scale studies suggest that removing the hormones can, in some individuals, slow metabolism. If that happens and an owner notices it all they have to do is adjust feed. Using mine as an example, Ike was neutered at age 1.5. He never gained a pound. Rita had her spay and about 3-4 months later started gaining weight with no change to her diet or her exercise. So, I knocked down her food allotment by 1/4 cup a feeding and after a few more weeks she was back in shape.