Because it is hard. When you build prototypes you don't need to deal with layout, touch interruption, loading data asynchronously, errors and so on. You design in the ideal world.
And to be honest today toolkits are not really designed for UI that you would like to achieve. Mostly due performance issues because you animate static images, but once you start animating stuff that needs to be recalculated and redrawn each frame it starts to be pretty complicated (and possibly slow).
Because it is hard. When you build prototypes you don't need to deal with layout, touch interruption, loading data asynchronously, errors and so on. You design in the ideal world.
And to be honest today toolkits are not really designed for UI that you would like to achieve. Mostly due performance issues because you animate static images, but once you start animating stuff that needs to be recalculated and redrawn each frame it starts to be pretty complicated (and possibly slow).