[Edited by moderator Latiosmaster47, 11/6/2009 10:19:57 PM]
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.
It's more fun with three other people though.
[Edited by Latiosmaster47, 11/5/2009 10:23:21 AM]