Valve believes that there is still a large market for single player driven games and not everyone wants the "stress" of competitive multiplayer.
In response to the "longevity" of single player games and "long-term monetization of customers" posed by Gamasutra, Portal 2's project manager Erik Johnson replied saying, "I still think the analysis that every product needs to be a competitor in multiplayer, or an MMO, is incorrect; there are a lot of people who want an experience without the stress, so I don't see that changing."
"You brought up piracy being a reason to not do single player, which I think is a pretty crazy analysis on an issue like that; that's making a decision for your customers about the types of products you are going to build without, by definition, including your customers in that at all.
"You're saying that because of these pirates, you get no single player experiences, which makes no sense to me. If there are as much players that want single player experiences, you should go build that. I think there are plenty of people that still want to have single player experiences. Look at Mario; those games do really well."
Johnson goes on to say that monetization of whatever title they make is a completely separate entity from their design process and philosophy.
"One thing to think about is, when we are building a game like Half-Life 2 or Portal, monetization is a separate thing that, in the context of the game design, doesn't make a huge amount of sense, really."