My thoughts? In addition to stuff like online passes, I think it's mainly because...games are "supposed to have them". You want to make a game, here's a standard list of features you need to include. Internet is popular so we have to include some sort of feature that implements it. Socialness is popular so if we can include that, more people will like it. And multiplayer just so happens to have all of those! Plus Call of Duty and Halo are super popular because they have multiplayer. So if we include all those features, the game will be super popular! I'm super duper serial.
I think it's less for nefarious reasons, and more that a lot of people consider those features to be standard features all games should have (although I totally get seeing it as nefarious). It's misguided sure (the standard I mean), but there you have it.
Ironically, all that bullshit about games becoming more and more expensive? I'd be willing to bet that a huge part of the reason they're becoming more and more expensive is because of forced online interaction.
You want to add online features to an otherwise offline game, that can be a boatload of work. Let alone having stuff like a proper lobby/matchmaking service, designing good maps, balancing the game regularly, fixing bugs, lots of playtesting, and on top of all that, somehow making it fun and unique. Then you probably also have to hire additional staff, networking programmers and whatnot. If the game wasn't designed with a good multiplayer experience in mind and instead its simply being included to meet the requirements or because it's the standard, then of course it's going to be crappy.
I imagine that's part of the reason so many games just default to Call of Duty competitive type multiplayer--it's the most popular multiplayer and the developers don't care for it or didn't give it much thought, so they figure they may as well just do that. The problem with including Call of Duty/Halo type multiplayer in a game that doesn't need it is that CoD multiplayer is tailored for a specific type of audience and experience(extreme competition), which might not necessarily mesh well with the audience of the actual game itself, along with the fact that in CoD and Halo, multiplayer was designed to be an ongoing thing and one of the core features. So when a game includes that type of multiplayer and the developers aren't prepared for it being an ongoing thing, it dies off after a really short time. Of course if the multiplayer doesn't mesh well with the game because it's a different style of game they could always...tweak...the design to make it work better for that type of multiplayer *cough* Dead Space 3
Essentially it's an on-going money sink that the budget has to take into account, therefore the budget goes up. But they have to include it because it's "standard". Some developer, I don't remember which, said something about the multiplayer in his game being a cancerous growth and that they had to include it for the only reason that the publisher said so.