Austin Harvey, who works at a bookstore says, "I see a lot of books. Most of them, I'm afraid, have terrible covers." They all look like the books in this collage, which he assembled for his Medium essay. He says it's a cheap marketing ploy:
You can find similar trends with thrillers, YA fantasy, romance, Christian fiction, and just about everything else. If an author isn't already well-established and unique, their book isn't going to be given an individualistic design. It's going to blend in with every other book on the shelf near it, the publisher likely hoping someone will say, "Ah, Divergent looks a lot like The Hunger Games, and my friend liked that a lot. I'll buy them this one as a gift. I'm sure they'll like it, too. Thanks, Katherine Tegen Books!"
Designer Divya Gurung digs deeper into this unfortunate phenomenon here: