That's because we've never had chance to question writers so much before and after a movie. I don't think he was ashamed of the script, he was sorry to see so many things didn't turn out as he had hoped, let alone as fans had hoped.
Full creative control is a rare thing.
Let's take a newspaper article for comparison. A reporter writes it, then several desk editors look over it and perhaps rewrite parts of it, then page designers lay it out on the page and alter it again, then proof-readers check it and might change it again.
Writing is not an untouchable final part of any commercial product. It's the starting point and sadly a lot of the original intention can get lost along the way. Sometimes things are improved, sometimes they aren't.