It comes down to user interface, not the product name or code base of the OS. One might as well ask, "Why aren't a motorcycle's operating controls the same as a car's?"
The realities of touch screen vs. keyboard/mouse/trackpad and larger screen vs. smaller screen drive this. Each interface method is sufficiently different that each deserves (nay, demands) an optimized experience. I wouldn't be surprised if, at some point, Siri is considered/branded an OS in its own right - speech-optimized control.
What good is the "efficiency" of having a one-size-fits-all OS, if the result is that nobody gets a good fit?
Then there's product identification/branding. When Microsoft insists on putting the Windows brand on everything, one of the messages is, "When you're using a smart phone or tablet, you're using a computer." There's a huge segment of the population for whom this is an utter turn-off. "I don't want a computer in my pocket, I hate computers!"
I get it, there are Mac users out there who wish that they had nothing to re-learn when they pick up an iPhone or iPad for the first time. There are Mac users who wish they had the depth and breadth of features and under-the-hood controls for iOS that they're used to with Mac. And they're grossly outnumbered by people who would hate any attempt to make their iPhones and iPads more Mac-like. The more likely impact of unification would be the simplification of OS X, to make it more accessible to the masses. Is that what any Mac user wants?
It's also true that there's cross-platform unification going on anyway, such as the multi-touch gestures on trackpads and MagicMouse. There's the "your data everywhere" unification of iCloud, apps of the same name and file format on both platforms... But equivalency does not require carbon-copy duplication - it requires consistency of terminology and functional equivalence, when functionality overlaps. And otherwise? Vive le difference!
