His reasons summed up for you.
Taken from his Extra Punctuation article: "Extra Punctuation: On Multiplayer"
1. He lives in Australia
"Australia is very far away from other English-speaking nations, which is usually a bonus because it's a lot easier to get along with people when you can't understand a word they're saying, but it does mean it's hard to find decent servers. So either you try to get by on a US server and put up with being issued some kind of strange delayed-reaction gun, or play on one of the few local Australian games you can find, which doesn't offer a lot of choice."
2. Time restrictions
" I'm a professional game critic, and that means I play a new game every week. Sometimes I regret that. Sometimes I'd like more time to explore the nooks and crannies, just like I did your mum. But this is the lifestyle I have chosen for myself and I can't really complain. Anyway, the thing about the multiplayer portion is that it's usually the bit that keeps the value for money going once the actual story, campaign-y, structured part has been exhausted. And I'm not usually around long enough to care."
3. Because there's nothing more to see
"I played World of Warcraft a few years back, and got quite addicted to it. . I got all the way to level 58, but then, like a flicking of a light switch, all my interest in playing further instantly evaporated. Why? Because I'd seen and explored every single territory on the world map. That's what had been keeping me going, I realized - the desire to always see what was over the next hill. And avenge myself on the giant spiders that had completely ripped my booty apart the last time I set foot on their turf. I really don't get people who can play end-game content and games like Counter-Strike over and over and over again. Nothing ever changes and nothing is ever achieved. And you're taking your own time away that you could be using to get to grips with the wonders of the unexplored worlds in the next game on the pile."
4. Because the single player must stand up by itself
"There was a very grim period of PC gaming, the whole Unreal Tournament, Quake 3: Arena era, when the emphasis was on multiplayer. This was thankfully ended by the arrival of a new generation of shooters like Half-Life, which reminded us what multiplayer is supposed to be: a nice bonus attached to an already strong single player experience. If the multiplayer ever becomes the main selling point, then something's gone wrong."
5. Because people are shit
"When you play online with someone, you're not a human being to them. You're just another little mewling voice in the magic box of secrets. If you're not in the same actual room, poised to punch them in the face, only their entertainment matters. You might as well just be an AI bot that swears. Surely playing against an actual AI bot would be preferable. They might not speak and get stuck in corners a lot, but at least they'll never ragequit, and you can program them not to shoot you, and you don't have to pay broadband internet fees for the privilege."