Pimp My Novel explained the book return policy book publishers extend to bookstores. A year or two ago, I was at a particularly interesting author gathering at which booksellers were speaking. One of them explained why she tries to avoid selling self-published books. She didn't talk about the quality of the writing or of the design/production. She talked about this book return policy.
When dealing with book publishers, she can purchase a number of titles from any one of them, return what she can't sell, and get credit for her next purchase. When dealing with individual self-published authors who have not found some kind of distributor, she can't. Or, if an author offered to take back the books and send her a check, think of the hassle. And multiply that hassle by 750,000, since that's the number of books that are supposed to have been self-published last year.
If every self-published book were a masterpiece, it would be mind-boggling for booksellers to deal with the bookwork involved in getting those books onto their shelves.
I believe some booksellers will accept some self-published titles on consignment if they are titles of high interest to their communities. But you can still see that the consignment model isn't the one they regularly work with and would be a nightmare to manage with a large volume of authors/titles.