Why are fatty tumors so common in dogs?

Why are older dogs so prone to fatty tumors? I (mistakenly) umed that if my dog had less fat on his body, his tumors would disappear. He's always been quite trim, but he has 2 movable, g-like lumps that the vet said are fatty tumors. What causes them, and are there any natural treatments to make them go away? I worry because the second one arrived shortly after the first, and if I don't know what's causing them, I can't prevent them. My dog is mixed breed, short-hair, 11 years old. Thanks!