It's still less content. You're just playing the same content over and over for more hours. Just because you'd play that amount of content for a longer amount of time doesn't mean that content costs even more to produce. You're still getting less actual content then a game that also has a single player aspect to it.
Of course they could change all that if it was a multiplayer game that takes the money it would have used for a single player campaign and uses it to make an even larger multiplayer aspect (more game modes/maps//ect then the average multiplayer game).
Oh, I see. You're defining "content" differently than I would. I think a good multiplayer game (e.g. Chess) can have virtually unlimited content, since the content is partially, significantly, or even majoratively defined by the community which plays it.
In that case, this is just another way of framing the "single player games cost more to make so their prices should be higher" argument.