compatibility layer = emulation
Whether you emulate an entire system or part of a system, you are still completing a hardware process in software, Q.E.D emulating.
The chips in both PS4 and XBO are significantly different enough from desktop processors that getting PS4/XBO games running on a PC would require several features and instruction sets to be emulated. This not really that different from older gen emulation and, as others have said, probably more difficult as these chips/features/instruction sets are far more complicated.