Not everyone has access to online play, or friends to hang out with. In fact, if you're playing enough video games to actually have full working knowledge of this situation, you probably don't have IRL friends. I know I don't. So if one buys a game, it better damn well stand on its single player. Now in games like Left 4 Dead, proud owner of the title "Only FPS Cupcaek Ninja will ever play because she still hates the controls to a point it's almost unhealthy", Multiplayer and Single player are the same thing. And that's fine in a game. If you want to make your single player completely the same as your multiplayer. But in a game like Halo, after the campaign is over, about 8 hours play time depending on the player, if you don't have access to online multiplayer, or just hate everyone on the internet(not an invalid philosiphy in itself), you've just wasted 60$ on a mediocre, 8-hour long game.
Most people seem to forget that video games are a way of NOT interacting with people in a social setting, and while multiplayer starts out as a non-interaction way of testing your skills against one another, it will inevitably lead either to a complex social situation like the ones you avoid, you know, OUTSIDE, with people you KNOW and who can SEE YOUR FACE, or it takes the opposite route and devolved into /b/, only less interesting. If you wanted that badly to socialize, you'd probably go outside.
Now do games without multiplayer work? Yes. Look at some old classics, Ocarina of Time, Sonic the Hedgehog Original, or the first Metroid. Or look at a few more modern examples, like the original Fable, or in the niche category, Harvest Moons Animal Parade and Tree of Tranquility. Or Fire Emblem.
That's not to say that all Single Player Games(SPG) are good. There's superman 64, or Sonic and the Secret Rings, or Sonic 3d Blast, or Wolfenstein(New). There are also Multiplayer games that are horrible. Who here remembers a game called Mario Party 8? Yes there are 8 of them now, possibly nine. I stopped caring a few turns into 4. Or how about those half assed Naruto fighting games there are so many of? Or those fantasy football games that there are like 20 of now per developer?
It's a simple matter of what is the most useful. If you bought Halo 3, brand new, and somehow lost access to connect with others online and couldn't get it back, for a ridiculous amount of time, you'd return the game, right? What about Bioshock? If you bought that, brand new, barely after the release, and lost multiplayer rights for whatever reason, it wouldn't matter. With multi-based games, if you lose internet, or get banned, or whatever, you've spent 60+ bucks on less than 8 hours of game time, in many cases. I don't care what the "price of entertainment" or whatnot is, that is just a bullshitty waste of money.
A good game is one you want to share with others, not one that makes you share with others to derive any entertainment at all. That's why Pokemon is still so popular. It has so many options, and it's fun to share your teams with others, but if you don't want to, you don't have to. You can sit quietly and enjoy your legitimately earned perfect IV shiny team you spent upwards of 2000 hours getting, without anyone else knowing, if that's what you want.
Think back on the "Perfect Game", Goldeneye. The multiplayer? was okay. What drew people to love the game? The actual game itself. The story, gameplay, the reason why you had to shoot the other guy. Goldeneye wasn't perfect because of mindless shooting of one another in vs mode, just to scream "screenwatcher" over and over again and argue about the weapons. It was perfect because of a story, gameplay, and characters that all bound together in the campaign to create the greatest experience known to gaming.
It mostly seems a matter of opinion, but Multiplayer games throw all story and characterization out the window, along with any immersion in a new world that isn't your own, which may I remind you, apart from preschool learning games, is the entire point of a game. If you wanted to be in your own world, you're already there. In the end you're just beating up some dude sitting in his own chair in front of the same machine that more or less looks exactly like you with the slumped posture and dull-to-IRL expression. That ruins everything right there.
Now if multiplayer is weaved into the story, like in Left 4 Dead, where Multiplayer is just Single Player, without (as many)AI teammates, you can keep the immersion factor. Yes the guy you're healing is sitting in a chair somewhere etc etc, but he's not. He isn't some nameless, faceless, gun toting drone that looks exactly like all the other ones you killed. He's Bill, or Francis, or Ellis, and he's part of your team and you need all of you to survive. These aren't blank sheets of paper, like in halo multiplayer, where everyone is faceless, known only by the screenname of those who maneuver him. These are people, with pasts, personalities, and friends. Now in a way, you could do this in halo, assigning personalities, pasts, whatnot, to the shoot-y, technicolor avatars of the people you play against, but it's kind of like assigning a personality to Cleverbot, or a Furby. It attempts to give the illusion of actually being something that thinks and feels, but really, it's just not and there's absolutely no way to think of them otherwise without becoming the guy that talks to his stuffed animal and pretends it talks back. As an adult.
If you'd rather play multiplayer that's your choice, your prerogative. But I can go on about this all day of why a game needs to have at very least a DECENT single player campaign, otherwise it's less of a game and more a high school hallway, except there are no teachers to stop the arguments and no lockers to push one another into to end the argument themselves.