It's this kind of thinking (on the part of their competition) that lets Apple keep their head start for way, way longer than necessary.
The short answer: Because what you're proposing would be terrible.
Apple's money magnet is creating a sense of delight and surprise. You'd be mimicking the result, a snap on cover, without grasping the intent, which is to make you feel like a kid seeing magic. People don't want a cover. They want to be delighted by a cover.
At least, they do now, since Apple has shown them it is possible to feel this way.