I'm not sure why, but I suspect it has to do with the fact that the story becomes real and therefore imperfect when the writing begins. Before I begin, I have this idea that the story will be compelling, entertaining, deeply moving, and yet clever and witty. But as soon as I begin writing it, I see holes and flaws and unresolved problems. I need to suppress the editor in me long enough to get the story out. What matters is to get your internal editor looking for the right things.
Language is nothing.
Of course, nine thousand won't be fine, and of those, about five thousand will truly suck. You can fiddle with language whenever you want. It has nothing to do with the writing process. What you need your inner editor to be concentrating on is what happens and why. Those things you need to be watching carefully, and those are the things that decide whether your story will feel important, true, and clear to the reader from the start.