You're not a software developer, are you? My code needs to run on 3.5" to the largest iPhone, plus iPads in three sizes, two screen orientations, and three ways of splitting the screen. That's not done by writing new code for every resolution, it's done by writing code that checks how big the screen is and adapting itself to the size.
You know how much effort it is for me to make my app run nicely on a 5.8", 5.9", 6.0", 6.1" and 6.5" iPhone, with different width/height ratio? I'll tell you: ZERO.
Well, I'm a designer, and I know that even though the app runs nicely across devices, something that's optimised for 4.7" might look like unpopulated empty crap on a 6.8" iPhone.