Because with the exception of lean (don't care) and the price (wait for a deal) none of those points apply to the single-player. If it delivers another "All Ghillied Up", then it'll be worth it for me.
I never once fired up the MP for CoD4, and never had any intention of doing so with MW2.
Well if you never fired up the multiplayer mode in Call of Duty 4, you are seriously missing out. You should give it a chance. It is easily the best part of the game. The single player was fun but it is the multiplayer part of the game that brings me back to play time and time again. After playing it for two years I'm in no mood to quit playing now.