I'll say this as someone who enjoyed both the multiplayer and single player aspects of Modern Warfare 3, and as a fan of Team Fortress 2.
It's not for everyone. People who don't get used to the mechanics or are fast enough get killed quickly, and people can get sick of it if they die enough times. The determined ones would grind for hours, but a lot of people will just through the Multiplayer portion aside and either go for the Single-Player game or another game entirely.
Plus, some people just want a decent story, and multiplayer doesn't present a story the same way as single player. Sure, you've got Left 4 Dead and Zombie mode in the Treyarch CoD games, but the fact that it's multiplayer imposes various restrictions on how to tell the story. It's not as cinematic and parts of it may have to be rushed; hindering the overall storyline.
Additionally, some people are happy with playing a game themselves, and you can't force them to do otherwise.
And I think the push for socially integrated games is detrimental to the genre as a whole. Look at the multiplayer-Sim City reboot coming out next year; while they let you play alone much of the game's economy is affected by all the players in the game. Plus, the imposing of multiplayer forced the Game Devs remove elements of the game that made the previous Sim City fun, i.e. terraforming, certain transportation options, and the ability to build large megacities.
Another example? Spec Ops: The Line. It's IMO one of the finest single-player stories since Bioshock, and while it wasn't perfect, it might've been better if the devs weren't forced to focus part of their efforts on the game's multiplayer portion, which even one of the Developers labeled as trash. There are games that only need a single-player mode and nothing more, and cramming a multiplayer mode into them will hinder more than help.
I'm not condemning multiplayer of course. I'm all for harmony between multiplayers and single-player games, and I think neither can possibly replace the other. To each to his own preferred style of play, and you can't force them to change their tastes.
To be fair, this is just one single line from a single person from Treyarch, so I think people are either rolling it into something exaggerated, or taking it out of context. Hopefully Vonderhaar will clarify his statements.