Because I subscribe to lots of people.
Be more you.
Interesting text to read.
L4D is a multiplayer game. I can't imagine playing it alone, and I don't see why anyone would.
Because the AI bots you play with in single-player are both smarter and more skilled than any real people you will encounter online.
The 10 year olds that populate the L4D servers have no concept of team-based gameplay.