My teacher who besides from teaching also worked part time as a graphic designer told me once that whenever she was hired to do a book cover, 9 times out of 10 she was given a large file full of stock photography to work with. She was free to do any design work or painting she liked but they were not prepared to pay her more for going the extra mile. So if this is common practice it's not really strange that so many bookcovers are extremely uninspired.
edit: Darn! I'm to busy to get stuck on that bad book covers site.
Although I did pick up a terrific quote.
"// Listen, Comic Sans is NEVER the right font choice. EVER"