I'm sure you are referring to the probably ghastly look of the body at that point, but there is a difference. This decomposition is because of natural causes; cremation is, as was said, unnatural and violent, a human obliteration of everything reminiscent of what the person was in life.
I guess someone could research this for us, but I'm quite sure that at least some states require a kind of container like a coffin to be burned with the body. It's not as elaborate or expensive as the casket, certainly, but it still results in body ashes and "casket" ashes being intermingled. In addition, there are quite a few people, I am finding, who want parts of themselves buried somewhere after cremation and the rest of their ashes scattered from a plane or on the seas, etc. That's open to being considered disrespectful, don't you agree?