Because I don't find Apple's hardware mediocre.
But the simple fact is that Windows is the platform game publishers and GPU / card manufacturers want to support. I can only assume Apple is actively denying hardware and OS support to the GPU and card manufacturers which is why we have never seen any third-party video cards for the Mac Pro like we did for the Power Macintosh.
And because Apple is denying that support to the hardware folks, it makes it very difficult for the software folks to port to OS X. Or the ports to OS X are in many cases not nearly as satisfying because of the conditions those authors are required to work under.
It also doesn't help that most of Apple's hardware is based on mobile parts to maintain the form-factors that make it desirable. Mobile parts trade performance for heat and power management (even if they never leave "the mains" like the iMac or Mac Mini) and they are not designed to be upgraded (as proposals like MXM and such never caught-on).
So until Apple decides to support video game hardware and software more openly, Windows is where it is going to be at. And since Steve Jobs has evidently said on many occasions he is not a fan of video gaming, I don't think Apple is going to make any serious efforts to be more open in the near-future.