It has to do with the fact that PC sales aren't guaranteed and are diminishing on many genres
Gears 1 on PC sold a *fraction* of what was expected, for instance
and PC development is more of a hassle as there are a LOT of additional compatability coding, tuning and testing needs.
Console development uses a single video/cpu/ram profile, PCs have TONS of profiles which each require optimization.
Casual Games (Popcap stuff, for instance), Sims and Wow-style games do better on PC and you've gots them.
And, of course, exclusive content is what drives people to the consoles, so if Fable II or Gears 2 were instantly available on the PC, there'd be less reason to pick up a console... even though a console's generally cheaper than a new vid card for a PC, some people just love handing ATI and Nvidia cash for the newest new.
That leaves out the fact that Piracy on the PC is *probably* more pervasive than on consoles.