It is obviously "hard to understand" for you sitting in that apple box, that they are comparable in many ways. But Motorola, Samsung and HTC think outside of your apple box and deliver various screen sizes in response to the consumer demand.Windowed operating systems are in no way comparable to a modal operating system. Is it really that hard to understand?
What about iPod? There are multiple screen sized ipods. Does the ipod use "Windowed" operating system?
Apple does not care what consumer wants instead trying to tell what consumer needs. Producing one phone model is simply cheaper and it keeps profit margins high - that is only a reason there is one iPhone. Eventually it will change but it will cost Apple lost sales.