Because it shows totally zero imagination from the company that would make such a thing, not a glimmer of a thought or idea would have gone into taking the same looking unit and simply stretching it out larger.
Any half decent designer would look at the requirement, 'A Larger Screen" and knowing it's a hand held device, and overall size is an issue, pockets, holding in hands etc etc.
You would carefully design a case/body to be as small and snug around the larger screen size as possible, without affecting functionality and any design aesthetics.
Someone like Jonny Ive did not get where he is today by simply making the same thing bigger overall. He's be a laughing stock is that is the amount of his design imagination for a larger screen product.