This is a good post by Kristine Kathryn Rusch about the ability of self-published writers to produce books of good quality. I think part of the problem for writers who haven't worked in publishing is that they view the publishing process as kind of a black box--you write the manuscript, it goes into a magical black box, and viola! out comes a book!
The human mind being what it is, the black box has a certain appeal--you don't have to learn (it's too hard for you, anyway), you don't have to trouble yourself, someone else will take care of everything for you.... Yeah, you've heard this before.
Of course the magical black box is a lie. Publishers hire people--and the people, not the publisher, are the ones who turn your manuscript into a book. It's not like publishers actually pay the vast majority of these people very much money, so even the people who work full time in book publishing routinely moonlight--which means you can hire them, too. To save even more money, many publishers rely on freelancers, who can work for you just as easily (maybe more easily if you're polite and you pay them on time).
Yes, when you do that, you have to be the boss and do some thinking. But at least if you feel that, say, your cover art isn't working, you're in a position to find something that works better--you don't just have to sit there and seethe.