Diatomaceous earth is not effective for internal parasites, that is a myth that just keeps going and going. Every environment is different, every flock is different. What works for one may not work for another. Many people do not have space to let chickens roam freely, or live in a place that restricts it. Or have far to many predators to be able to safely do that. Eating earthworms, grubs and other insect larvae is completely different than having internal parasites. In places where the parasite load in the environment is heavy, worming is really not optional if you want to have healthy birds.