One of the biggest reasons is because they are not really big on the whole "upgrading your PC to play the latest game" business structure.

Their whole concept of buying a mac is so you don't need to upgrade your PC every few months to run applications. For PC gamers, they WANT to upgrade their components every few months to get the best graphics and gameplay. Take Crysis...... almost every PC gamer had to upgrade either their processor or graphics card or RAM (or all 3) in preparation to play it. Also, Apple wants things to be as unified as they possibly can. With the hundreds of variants of hardware for the PC, everyone is bound to run into Hardware/Software incompatibility somewhere down the line. Elsewhere Apple wants to offer a problem free computer out of the box and able to run without any hassles. That's their whole marketing strategy.

Apple doesn't want to open their platform to 3rd party hardware vendors, because their main business model is hardware first, software second. So if their rice bowl is the excellent build, yet expensive hardware parts, why would they give the license out to outside vendors?