"while your first question may be the most pertinent, you may or may not realize it is also the most irrelevant" -The Architect
The UI is the last, and most cosmetic layer in the OS stack, and consequently the one that's the most easily replaced. Some manifestations of the OS don't even bother with it at all.
Indeed! And hence my sad frustration with the failure of Windows 8 to catch on. The underlying OS was fine, in fact it was an improvement over Win7 in some ways (speed, footprint). If only Microsoft had not been so headstrong about destroying the old UI, all would have been well.
As as you say, the UI the most easily replaced component -- yet the vast majority of Windows users never change it. They barely customize their environment at all. Damn few ever customized Win7, or XP before that. They suffer with the default, whatever somebody gives them.
I think most users treat their computers like their cars. Damn few consumers customize their cars. So imagine if a major car maker came out one year with a total change to the car's UI -- say, they took out the steering wheel and replaced it with a joystick -- you know what the reaction of the consumers would be.
It wouldn't matter how "COOL" or "MODERN" it was. Nobody but the fanatics would put up with it.