The best response I saw there was that it's simply become fashion. Apps change for the sake of changing. The same is true for cars so that people can tell that

The best response I saw there was that it's simply become fashion. Apps change for the sake of changing. The same is true for cars so that people can tell that the older model is older and the newer one is different. It's also in Apple's designs, square edges -> rounded -> sharp edges, thin wedge, slab, etc.