I thought this was a perfect reasoning why there's no shadows in iOS:7
Source: https://medium.com/design-ux/e5b7bf3318e8
I think theres a simple answer to this, and it isnt about iOS going flat. My impression of the illumination metaphor (for lack of a better word) in iOS 7 is that the whole user interface is backlit, just like the physical device screen is. Hence, no shadows.
Source: https://medium.com/design-ux/e5b7bf3318e8