Well, I'd imagine the bottom line is that single player games take more effort, as opposed to creating and (maybe) testing multi-player maps. You could spend millions of dollars producing a few hours of single player gameplay that's entertaining, but in comparison multiplayer content you can sell for $15 a pop can be produced for a fraction of that.
It doesn't surprise me that the guys doing shooters kind of wish they could just skip over the single player (although he's not coming out and saying that) which would mean less work for as much or more money.
That said, not everyone who plays video games has a game playing peer group, all stereotypes aside, a lot of gamers are very solitary. What's more breaking into the needed social infrastructure to do well in multiplayer is not easy due to the elitism and people preferring to stay with their own closed little groups. Then of course you have all the so called "trash talkers" and little kids who get into these games and act obnoxiously.
See, the "human factor" which is the appeal for some people, is also a turn off for others. No amount of training or tutorial practice is going to deal with the negative aspects of dealing with other people.
To be honest it's been my experience that with the way how games have been catering to the lowest human denominatory and the casual audience, that those who are into multiplayer games tend to play them with other people they already know IRL. The whole "Bro Pack" and associated jokes in coming strips and such exists for a reason, and really not everyone wants to deal with that to begin with, and isn't going to insinuate themselves into a peer group to begin with.
Keep in mind that being a lonely, basement dwelling nerd is NOT the only reason to be solitary, though I fit that description nowadays. Back when I was working casino security I had little in the way of a social life because working a real job meant that I wasn't there for just 8 hours guaranteed (I couldn't leave until relieved, if there was nobody to relieve me due to shorts on another shift or special assignments... well), travel time, and then of course exhaustian and stress. I sat down then, especially then, to get the heck away from people, that was kind of the point of gaming. I don't mind multiplayer games (I'm big into MMOs, and was even when I was working to an extent) but that's not something I want all the bloody time.... the guys doing "Black Ops" aren't the first to talk about wanting "multi-player all the time" to be the future of gaming, and really I just hope that whole idea dies. I get concerned that if enough developers/publishers go on about it, they will convince themselves it's what everyone wants, and then make it happen despite what the reality might be.