Millions of people buy the game every year because $65 is actually pocket change for some people and they'd much rather have the most recent iteration to play with their friends. It's a fad. And, whether you agree or disagree, CoD has the potential to give out at least 100-200 hours of gameplay due to the multiplayer.
$65 is pocket change to a small percentage of Americans, especially in this economy. Then again, that's not counting the kids with jobs who live with their parents and get to blow their paychecks every pay period.