Because stock iOS (iOS 7 changes this a little bit, but not much) only gets you so far. Things like pixate get you a little further, but ultimately some sort of visual design process comes into play, typically with a tool that's been used in that process since time itself began.

Background: 20 years as a creative technologist currently doing iOS based experiential retailing for huge brands where the interactivity and visual appeal simply doesn't exist in stock iOS. And I also wrote an app publishing tool that works similarly to Adobe DPS (build informational apps in Photoshop, see http://www.youtube.com/watch?v=YXlHFhbqzHU).