Think about it this way. Often, when a book has been published, that means the book itself is good, has a good topic, the writing is amazing or all in all, the plot is good. However, just because a writer can write, doesn't mean they can describe. A writer is the one asked to come up with the blurb on the back of the book, are they not? Well, if they are not good at describing things--or their work in particular--how are they supposed to come up with a good description of what the book is about?
Now, I realize authors typically have very little to do with the cover art. That's the job of the publisher to find someone to illustrate the cover and make sure it's good. And if they don't do their job, obviously the author isn't really allowed to do their job either. Here's how things work: let's say a teenage girl walks into a bookstore. She goes directly for the young adult section. The first thing she looks for is a cover that makes her want to read the book (i.e. an eyecatching, brightly coloured or mysterious cover). When she finds a book that has a cover that interests her, she reads the back of the book. Now a couple of different things can happen. One, she may return to book to the shelf because the blurb does not sound interesting. Or two, she may decide to buy it because it sounds like a good book. Either option is possible. However, no matter what she does, the book could still be good or bad. Just because it has a good sounding blurb and a nice cover doesn't mean it's a good book, right? Of course it's great marketing, but truly nothing is for sure until the book has been read.
How many good books are we missing out on because of poor marketing? We could be missing out on hundreds of amazing books that hang below the radar or which are done by unknown authors who do not have the marketing skills to promote their books. Have you ever thought about that?