Writing Journey: Why Does It Take So Long?

I'm not talking about the eight years of writing, submissions, and rejections I went through before landing my first novel contract, although that did take forever.
(Granted, I was publishing articles during those eight years, so I wasn't a complete failure . . . I just felt like one.)
Today's topic is a reader question I got from Chas Hathaway some time ago:
Why is it exactly that it takes so long (2 yrs I've heard is average) for a book to get from submission to publication? Is it editing...