Assassin's Creed: Brotherhood and it's fucking brilliant multiplayer alongside its wonderful single player would like you to understand that that's not always the case.
And yet, it is the case most of the times.
A multiplayer portion will require a daunting amount of resources (including time), the engine doesn't seem very fit for a high frame gameplay experience, and more to the point: Mass Effect has been built around the single player experience crowd in mind.
I could not care less even if the multiplayer portion was built to be a masterpiece.
And I'm not even a misanthrope.
The cold, hard fact is that I do not go to Mass Effect to be competitive, meet new people, or do the same repetitive task over and over again until I lose my mind for the same reason I do not go to the gym to order a hamburger.
We already have great games for that.
Moreover, I think it would be better if you build a game to be multiplayer from scratch: that way you don't have to bend over backwards the story just to fit some marketing* director asinine sellout concept. Why can't a game remain specialized to an area?
Better to be King of something than a Jack of all trades.
* or whatever other division.