Why? They set the tone for technology in consoles. I forget who made the quote, but a few years back someone in the industry said that PCs will always have a place because while they're continuously improving, consoles are only a snapshot in time of the best hardware. They cannot be any better than that.
Some of the best console developers came from making PC games. (This is a source of angst for me when they stop supporting the PC, but nonetheless) The development platform for anything is the PC. FPS's are inherently better. RPGs tend to have more depth on the PC. Overall, memory and graphics advantages on the PC make it (at least in view of sheer prowess) a natural choice for gaming.
You think Valve will stop making games for the PC with the sales of HL2? We will continue to see more cross-platform games, as is the trend, and we'll continue to see the occasional defection like this Ghost Recon bullshit but making sure games hit the PC is a natural choice for anyone wanting to maximize their profit.