The key difference that Resistance 3 and Gears of War 3 have in comparison to Mass Effect is that Mass Effect has never taken place on Earth before ME3 where Gears of War and Resistance all take place on Earth.
Well, they take place on a planet. But Gears of War
takes place on Sera - I don't even think Earth is ever referenced in the canon.
Well, in that case props to them for not going back to Earth.