You can put as many cpus and cores and videocards into a box, slap a price tag on it and call it an insane deal.
But if it doesn't run OS X, what good is it?
User Experience, User Experience, User Experience. At the end of the day, THIS is what separates beauty and function from cobbled-together shovelware. THIS is why Apple makes insane amounts of money, bad economy or good economy. THIS is what makes for record quarters.
The entire point of a Mac is to gain access to and experience the Apple software ecosystem. The beautifully-designed hardware is simply a bonus, but also fully in line and complementary to that software ecosystem.
I can probably build a PC, case, fans heatsink, parts and all, that'll best the top-end Mac Pro in raw specs/peformance. But why would I want to?