Renowned book designer Chip Kidd (who designed the cover to Jurassic Park and many others) once said that a good book design is about giving form to content but also appreciating the balance between the two. Book cover design might turn out to be a very complicated process. While the designer is after creating a catchy, bold and unique identity for your book, you will want as much of the story to be told through the design. Just because of this, there are many reasons a pre-made book covers might be the best idea for your forthcoming book.
You will get a unique design, not specifically done with your story in mind, but still a cover that has been worked on from scratch and one that you have have found be fit to tell your story. This cover has also been made sure to convey all the different aspects of great book cover designs. Secondly you will know what you're paying for. A great idea might not always turn into a great design (or more specifically the vision you had in mind might look different from what the designer will create). So there's always an element of risk-taking when paying for a unique design. Many authors prefers pre-made covers just because of this. They know what they're paying for, before making any payment. And then there's the element of price. The reason you will usually pay hundreds (if not thousands) of dollars for a good design is because you will have many revision rounds. These takes time and effort on the designers side and naturally he or she wants to get paid for the extra miles. However when you buy a premade book cover you only pay for what you see and don't have to pay for the numerous revision rounds involved (hence the lower price).