It's rather simple why they don't allow it in my mind. A screen full of semi-uncontrollable app icons becomes a marketing piece for Apple. If you see a person out and about using their phone and you see their screen chock full of icons, you know it's an iPhone. Apple is very much about having their devices look the way they want so that they are recognizable in all facets, including software. If they did allow you to arrange your home screen however you want, what would set it apart from a competing Android to the glancing eye? It's the same reason they have yet to allow third-party watch faces. They control, and in turn limit, the content so that it aligns with the look and feel of what is shown in their marketing materials. This may be an outlandish opinion to some, but in a lot of ways, the consumer becomes a controlled marketing tool for Apple.
