I don't know. An iPad with cellular is hardware-wise arguably more similar to an iPhone than to an iPod touch. Software-wise, it doesn't have Phone.app, of course.

So I suppose that's maybe the key: whereas iPads and iPod touches are always meant to be held at arms length, iPhones are additionally expected to be held against your ear and are shaped slightly differently because of that additional use case.

Or maybe it'd be hard to make a "half-pipe" from aluminum on a device as thin as an iPod touch.

Or maybe it's all just arbitrary and everything in life is random and meaningless.