I don't think its quite as clear as that although I'm sure money is a HUGE factor. AMD had to collaborate with Apple for their proprietary single tile 5k P3 display TCON. We don't know much more than that, for all we know nVidia could have said to buzz off.
AMD and nVidia have different business models. Neither AMD nor Apple are currently struggling especially AMD with their contracts. I mean come on, you are 5x more likely to see/use an AMD product than nVidia in your everyday routines....Playstation and Xbox alone almost guarantee that.
You just feel nVidia is more premium because they currently have a better GPU line out. And with the use of Vulkan those lines are being more and more blurred. The Fury X was AMD's version of the Titan X (previous generation) and it held its own.
This is reminiscent of saying the Galaxy S(whatever) is better then the iPhone simply because they have different launch dates. Of course the newest thing from either competing company is going to be better.
List your parts you are using....
Its literally impossible to build a computer that is 4x faster than a maxed out iMac at the same price. Even if you are doing something as weird as comparing it to last years iMac you still can't build a PC that is 4x faster at the same price. You'll exceed the entire price of the iMac in a quest finding a CPU that is 4x faster. The iMac uses m.2 PCIe storage, nothing that is 4x faster. Thunderbolt, USB, etc etc all similar. Memory can definitely really be improved but not 4x...maybe 2x. So really when you say "pretty much every aspect" you mean "only the GPU"?
Plus you are building a desktop tower not an AIO which the two shouldn't be compared so easily. And YOU are building it yourself if I build something equivalent to a maxed out iMac with a GTX1080 through another manufacturer I come up with this. (its hard to find everything Apple offers in a BTO pc especially a 6700k and m.2 storage but I found Alienware has something similar).
i7 6700k, 32gb RAM, 1tb M.2 SSD, Windows 10 Home 64bit, GTX1080 Founders, with an optical drive (I couldn't remove that so take off 50 bucks).
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No monitor, no speakers, no mic, no camera, no thunderbolt. (I'm ASSUMING it comes with a mouse and keyboard although not positive about that).