I'd like to point out, first and foremost, your justification for Mass Effect's and Gears of War's reasons (i.e. "magnets") kind of applies for the other games you mentioned. In other words, "magic" instead of "magnets."
As for a more realistic reason, it's an aesthetic choice. The designers spend a lot of time making pretty things, they want to show those pretty things to the gamer. But if you only ever have the sword drawn in combat, at which point it's moving around and is inside enemies a majority of the time, then it sits in an ugly brown scabbard for the rest of the time. And I don't know about you, but as far as my experience goes, you can't really make a brown scabbard pretty, no matter how hard you try.
As for why it's on the back? Two reasons. Let's use Dragon Age 2 as an example (first one that came to mind). First reason: Greatswords and the like would not fit on your hip. They're as tall as your character is. Regardless of the practicality of actually USING a sword like that, if they put it on the hip, the model would pass through the floor unless your character spends the entire time levitating. Why do they use this for all the equipment, then, if only greatswords and axes and whatnot would pass through the floor? Because having all the models go to the same place on the character makes the programming easier than having a separate place for each weapon type. It also make it easier to animate, because no matter what weapon you're using, you can use the same animation to draw and return it. Second reason is, again, aesthetics. What part of the model do you spend most of the game looking at? The back. Where, if they wanted to show off the weapon models the most, would you put the weapon, in that case? Again, the back. After all, not many people spend a majority of the game staring at their player character's hip.
All in all, yes, I understand where you're coming from, but it honestly makes sense, given the context. That, and like I said, it's MAGIC! *jazz hands*
