Except that OSX does run on PC's. The architectures are identical now. My MacBook Pro can run windows natively because it is an x86/x64 architecture from the ground up.
I am aware of the architecture of Mac hardware. But OS X doesn't have driver (thus hardware) support for all (or even a large range of) hardware that conform to the architecture. Apple develops (espcially when it comes to optimizations) OS X expressly for it's own selection of (as opposed to general purpose) hardware. Apple also goes so far as to actively try to prevent OS X running on non-Mac hardware[0]. Contrast this with Linux, *BSDs that are developed on and for a wide range of (and general purpose) hardware.
