Why? Because based on rumors, patents, and the decisions they've made and the actions they've taken (or not taken), it appears as though that Apple believes that in the not so distant future the concept of having a separate mobile device and a computing device will be irrelevant. Consumers will have a single device, and that device will be capable of some form of altered reality interaction, or at least some impressive context switching behaviour.
It's already the case that an entire generation has been raised with their primary computing experience not being on general computing devices, but on some variant of mobile or tablet computing. It's not too much of a stretch to consider that they will be most comfortable with a device that behaves with some familiarity to that experience, but which can engage in much greater utility in a pinch. And it will be mobile, and it will require a better input experience than mashing your fingers on the small screen that splits its available space between the important information and large input elements.