How tough would it be for Bush to show he had been there? One person who remembered him being there? Where did he live while he was serving there? Did he have any friends? One official document saying that he showed up? How hard could it be?
Silence can speak volumes.
Can you show me a document proving where you were on march 3 1972 (better yet make it an official document)? How about a friend that can vouch for you? It was a long time ago! If there was real evidence of this it would have been used instead of having to forge documents.