nobelharvardsMy guess, is either because Windows 7 does not play nice with 8 core CPUs, since Windows 7 was released back in 2009, or...
AMD have always been the underdog against Intel. Because they have less money, I am guessing they are forced to focus more on the latest OS drivers (Windows 8, 8.1, possibly 10 as well), and less on the older ones such as XP, Vista or 7.
I think it's probably something more to do with multicore optimizations. AMD was fine compared to Intel in the Phenom 2 days. FX CPUs are historically bad at single-threaded operations.
Remember, even DX11 is single threaded.