Statistically 70% of the gaming market is exclusively single player. I know no one believes that, probably because forum posting sorts are likely savvy when it comes to linking things together, and likely more community based. Add that to the media overload on the multiplayer front, then no one pays attention to market statistics preferring not to try and figure things out on their own.
But for the multiplayer dickwaddery.
1. Some people enjoy griefing: A griefer is a defined player type, one of eight I believe (it has been a while since I delved into that part of game theory)
2. Some people over react: Gaming is inherently designed to be frustrating. If it weren't it would be too easy and therefore boring. So you are going to be tense, and you will eventually lose. In balanced multiplayer scenarios you can be assured of losing about half the time. There are very few other areas of polite society that throw this much dirt into anyone's face. As such the governances of polite society tend to be broken down very quickly and hate filled reactions and poor sportsmanship rear their ugly heads.
3. The whole anonymous thing: You'll probably never see these people again, they don't know who you are, etc. etc. etc. This is the reason I go by mark_n_b, I want to say "I'm accountable"
4. mixed skill levels: This is why I, and most people, don't go multiplayer. Shit-talk is easily handled and a fun part of the experience, unless you are trying out a new game and there are two other players who have been playing since Beta, one is always killing you and then doing the idiot tea bagging thing, the other is on your team calling you the worst things he can type because you are not an instant expert. And you are neither learning how to play the game better (because you're dead most of the time) nor having fun (same reason).
5. It's easier to remember the crap: Honestly, most of the people you encounter in online multiplayer are there to have fun and could care less about the score or winning or losing or who's new or who's old or whatever. But one bad apple does spoil the bunch and after five games you are going to remember how unpleasant that one guy who spent the whole game camping spawn points and that other guy who called you a "*****************" before you'll remember how much of the map you learned, or how you got a "good job" message sent to you after a successful push.