With simple icons, like the Phone app, the background is not overly distracting. Unfortunately many apps believe they need to fill the entire icon with detail. The YouTube and Notes apps have pictures instead of simple shapes. There is nothing to subconciously recognize so instead I must interpret the picture and conciously map that to the name and purpose of the app.
Apple could have used spacial awareness to help us recognize apps. Unfortunately iOS messes this up too, the home button can take me to any of my app screens. So I need to get my bearings before hunting for my app.
We are left with just color and patterns to differentiate icons. This makes the task of recognizing apps much more difficult purely so the phones look nice in store.
It seems likely from your reply that you have no education in matters related to vision. The human visual system uses other things too, yes, but silhouettes are one of the most dominant features because they are most invariant to lighting conditions. (Think about what would happen if you mostly depended on color, but now you are walking around at night when humans have poor color discrimination).
