Remember that Apple, like every single other company out there, has the single purpose of maximizing its profits for its shareholders. Everything else is irrelevant. If you believe otherwise, you need to learn some history.
And the way they do that is by making products people want to buy. Running the same UI on both a Mac, tablet, and phone will result in a sub par experience on all three devices and will surely drive away users. iPhones have no business running a windowed GUI, and Macs need to be able to have multiple windows up due to their expanded screens space. Macs use indirect interaction with highly precise input devices, while iPhones and iPads are direct interaction and have a fairly imprecise input mechanism. So many fundamentals are so different that merging the two UIs would make their devices desirable to no one. Not to mention the fact that iPhones have many more sensors than a Mac. If Apple wanted to pursue your strategy, they wouldn't do it by merging their operating systems. They would do it by locking down OS X while still keeping it as a separate platform.