Mass Effect started as a single-player RPG series, and Mass Effect 3 is marketed as a single-player RPG.
Multiplayer gaming is decidedly different from single-player gaming. Part of that is the social element, but that's not necessarily core to the experience. The other part is that multiplayer is inherently repetitive. In a single-player game you go through a linear series of areas once, maybe twice. In each area you accomplish a certain goal. In a single-player game that fails to differentiate the areas, or repeats them too much, or repeats the objectives too much - essentially, one where the experience fails to change - people will complain. On the other hand, repetition is key to multiplayer. You cycle through the same handful of maps, striving for the same handful of objectives, for a theoretically unlimited amount of time. The addition of human players is supposed to liven things up by adding unpredictability to the mix, but honestly, I don't think it does. Sometimes you get some real creativity, but when you're decent and play with other people who are decent, every encounter just becomes a crapshoot as to who walks away. Obviously skill comes into it, and it's challenging, but it's extremely repetitive. Also, anyone who thinks the addition of mouthbreathers with headset mics makes things better is welcome to their fucking multiplayer horseshit, just get your stupid grind out of my immersive, player-driven space opera.