I want to do more than win a match, I want to beat the game. You can't beat multiplayer.

Also, singleplayer exists to entertain me personally. I can pause, quit, restart, mod, cheat, and engage in completely counterproductive nonsense whenever I like. I don't have to worry about game balance, fairness and making sure the computer has fun.

Also, while I'm sure a majority of people in multiplayer aren't assholes, it can seem that way when the assholes are the only ones who do anything but silently play the game.