It's very easy to explain. Let me do it. Here are the best reasons I can give, roughly in order of importance.
Publishers think multiplayer sells games, so they demand almost every game has multiplayer by default, unless it is incompatible with the genre. It's a bullet point and despite successes of single-player games, evidence suggests it is still games with compelling multiplayer options that sell. Because of this demand, developers often have to divert attention away from the single-player portion of the game, reducing its quality, or even splitting their team in half to accommodate the two portions.
Games are getting more and more expensive to create. Multiplayer provides hours and hours of fun for comparatively little effort or cost. You give the players some maps and game modes and so long as it's fun, that's a guaranteed "X hours of gameplay".
Microtransactions and downloadable content are two of the best ways to make money these days on a game, because most big-budget titles don't even break even. If you can develop about 1% man-hours' worth of content in as much time, but sell it for 10% of the price of a full game, and not have to deal with major distribution costs or third-parties like retailers, that represents an absolutely massive potential profit margin. Although microtransactions and downloadable content work in both multiplayer and single-player, multiplayer is much easier because it usually involves little more than a few weeks of work to get a few new maps up and running (often built from single-player templates, cut from the final product due to time constrains, or reused from previous games), or a new gun, or whatever. Single-player stuff tends to require art, design, and programming, and fairly large teams on each front, while multiplayer stuff can be built with only a few people, usually an artist or two, one level designer, and a programmer or two to work the new features into the existing UI and handle the distribution process. Put simply, it's easy and cost-effective.
Multiplayer gives a reason for players to keep coming back for more. If you have a lot of people playing your game, that means they are less likely to buy what the competition is offering. Furthermore, online games are the ones people talk about. Because of the social element, players and press alike are probably going to promote those sorts of games more, both in face-to-face interaction and journalistic reviews.
Finally, developers create multiplayer while operating under the logic of 1+1=2, that is to say, if a game is fun with one person, it will be exponentially more fun with more people. Simple really. This isn't always the case, but I have seen this mentality before, especially with MMORPGs and similar.
I almost always prefer single-player games, although multiplayer is good for killing time and can provide certain experiences you can't get in a single-player game simply due to the constrains of artificial intelligence and the like. However, I have to admit that the above points have merit, even if it is mostly economic. Gaming is becoming increasingly mainstream, and generally the mainstream is comprised of more than a bunch of cynical, lonely, world-hating nerds; that is, the kinds of people who like to socialise. As this happens, demand for multiplayer goes up, although it doesn't actually represent as big a market as some would like to believe (most gaming is still casual and/or portable in nature). It's where the money is, and the fact is most people do not want to really devote even 20 hours these days into beating a game for what they may perceive as little to no reward.
I'd love every game to be as good as Deus Ex, to provide as engrossing and open-ended an experience as Oblivion, and as strong a storyline as The Witcher, but it's not going to happen. We're probably going to see gradually less of those games, too, although they certainly will never disappear completely, because even if it isn't the primary market, it still is a large enough segment worth considering. I think indie games are really going to take off in the single-player arena, with increasingly big-budget games moving into the online arena.
