Yes, you need to ask why. What is it that makes one camera perform vastly better than another?
Well, the price for one thing. If the 1DX didn't do anything better than the 5dIII, nobody would buy it for twice the price. So its understandable that Canon would use certain items that are better in the 1DX to differentiate it.
Why are Sony sensors better while the cameras using them are cheaper?
Because a sensor is one item in a camera, and the best sensor doesnt change a blurry or missed shot. Also, if we're talking video, I'm not sure I'd agree the Sony sensor is better (reviews pretty universally favor the Canon footage). But for photos, I'd agree that the Sony sensor has some potential advantages
Also, its coming from different companies...so maybe Nikon is taking a loss on the D800 to get people to convert to their system (Sony sold the PS3 at a lost for a few years, for example). Maybe the D800 has really a really cheap build. Maybe Canon is fleecing their customers. I'm not sure you can answer that question in any reasonable way since it involves multiple companies, multiple strategies, and thousands of different parts.
Why does the 7D use the same sensor as the 550D? Because the price is not in the sensor and the 7D doesn't make better pictures once the light gets through to the sensor than the 550D.
And the answer there is easy, as just reading the spec sheet would show you all the differences in the camera. Again, the best sensor in the world doesn't matter if the object is out of focus, or if the camera can't process the shots fast enough to even take the shot.