I have a reasonable amount of experience of the publishing world: I worked as a non-fiction editor for a book-packaging company which gave me direct experience of editing for some of the best publishing houses in the world. In the process, I learned a little about the publishing business; the differences between good and publishable; and the horrible truth about the slush pile.
I've had a reasonable amount of non-fiction published and so have seen that it is possible for complete unknowns to get themselves good publishing deals with a little bit of luck and a lot of hard work.
I've also had all of the novels I've written so far rejected. I've won all sorts of prizes for my fiction and have received only positive comments from agents and editors, so I know I'm competent: but what went wrong?
While I'll agree that my second novel is overlong and far too quiet, I still consider my first to be good-to-excellent—but, having worked in publishing for so many years, I can understand why it hasn't been published, despite a few very near misses: it would have been very difficult for the sales reps to sell it into bookshops.
Without my editorial experience I would be far less able to understand why that's so important; and without my non-fiction publications I might have gone on to conclude that it's impossible for a newcomer to get published. I'm lucky: I can see this from all sides and while I would dearly love to see my novels in print, I can understand why they are not.
I can only imagine how painful it must be for good writers without similar industry experience to understand why their excellent work has been rejected.