Also, the idea of "planned obscolescence" goes out the window when you realize something:
We used to get nothing but SP games. SP games are PERFECT for this because of the same reason books are perfect. People generally won't want to re-play/re-read the same story hundreds of time (I am not most people :p).
Now? Now most games are multiplayer first (or primarily) with SP considered "training". And the only way to make a multiplayer game obsolete is to make a superior sequel. Otherwise, I'll keep playing CTF-Face, thank you very much.