I suspect the main driver for a "flat" appearance is that basic shapes are easy to implement across screen sizes (e.g. a colored rectangle scales pretty trivial

I suspect the main driver for a "flat" appearance is that basic shapes are easy to implement across screen sizes (e.g. a colored rectangle scales pretty trivially; something like a fancy gradient may require a lot more effort to look reasonable on 14 different device sizes, especially if icon artwork is involved for all the corner and side pieces). Also, clear icons are easier to construct with simple shapes than with detailed pictures, and symbolic icons may localize better. For example, a highly detailed picture of a mailbox varies between a lot of countries; a symbol of an envelope may not, and it's probably more obvious anyway. Personally, I think a nice middle ground was the original iOS, where you could do cool things like define simple shapes for tab-view icons but have the OS stylize them for you (i.e. a plain white shape would end up with a cool blue gradient and look perfect in the black gradient tab bar, even though the original icon had no gradients or colors or rounded corners). And the UI elements, while sometimes a bit complex, managed to scale just fine between the screen sizes of the time.