Exactly. The touch paradigm is completely different than the mouse and keyboard one, phone usage patterns are completely different than that of a Mac, the distance from the screen is different, proportions are all different... Imagine looking at a Mac application, that has to be displayed in ultra high resolution in a small phone, in direct sun or while running and trying to click that 3x3 millimeters button to close the window.

An application's phone interface has to be different than the desktop one, and since there is no programmatic way to adapt the desktop interface to the phone, it has to be rethought and then reimplemented.