Because, for example, I'd prefer to travel with a single device and the MacBook is better than the iPad at certain things and vice versa. It's not that big an imposition to take two devices to be sure--and I'm sure Apple doesn't mind the extra sales. And I can mostly sub my phone and maybe a Kindle if I'm trying to go light. But it feels as if there's the potential for a unified device someday there--if it can be without compromises.