your PC may own all but with the fact that game developers don't like making PC games as they make next to nothing due to PC games being all to easy to pirate and the for the average Joe to use, the PC game market is dying and the game titles you can get do not own all!
it is mainstream market is dying, online games are better then ever but offline story mode based games are dying hence the reason they are spending more time on the online parts of games then offline story modes
Halo 2...which was released over 2 years later than the Xbox version... It's been over a year since Halo 3 was released and no sign of a version for windows... I think thats strong proof that Microsoft doesn't consider the PC game market as important...