When someone notices that they have made an error and so they edit the post and press stumbit or whatever, then they don't need to see it one more time. I think, it would introduce an unnecessary step. Let's keep the site the same. I like the way it works. :)
If your premise were correct, then there wouldn't be many instances of confused, repeated updates. I work that preview button when I have it, and many times I don't know what it's going to look like before I paste in what I have. Hold on, I'm gonna preview this before I hit create....