Mainly because it being a single device that fits in my pocket is really useful and convenient. I don’t want to carry around a tablet in a big with me at all times - I want to have one available by flipping my phone open. It also avoids the digital separation of two devices with different data, interfaces, etc. And it’s a convenient, easy experience. I can view something on the small screen, and if it needs more attention or scrutiny, open it up and view it on the big screen.