I think most authors don't self publish for three reasons.
- Publishers have the ability to get physical books into bookstores.
- (related to point one) The publishers sell the books to the bookstores, however if the bookstore can't sell the book the publisher will buy it back. Nevertheless, the initial selling of the book is done by the publishers to the bookstores.
- A lot of authors like the advances that publishers provide.
I really don't think authors are going to a publisher for the publisher's editing services, cover art, etc. It' all about the publisher's ability to move books and make the author money.