The book covers the most crucial marketing factor to your book's success. You're mistaken if you believe that books are not judged by their covers. Book covers create the first impression for a potential buyer or reader. It helps sell a book by making the contents immediately known.
Of course, the cover can create confusion. That is partly why publishers started putting the words “A Novel” on the book cover because they realized that sometimes people needed to be reassured that the book was something they were familiar with.
Publishers recognize the value of creating that first impression to ensure a book's success. When using larger publishers, authors rarely have the right to have approval over the covers and even have to work to get the right for consultation in their contracts.
Who designs book covers? The answer to that question is that with larger professional publishers, the author does not create the surface and the ideas and goals from the body come from a publisher invested in selling the book.