Why do people say that gluten free casein free diet doesn't ever help autism?

I hear people say that it wont help and never even try it. But I was watching doctor oz the other day and it had many doctors who treat people with autism all the time and many of them were agreeing that gluten free casein diet has helped many of their patients and that it's worth a try for everyone with autism. But I don't get is why people say that it absolutely won't help if they have never tried it? Why are people like this