I know because seeing your feet is the highest selling point of any video game. I'm always looking at them. I rarely look up to aim. I just stare at my feet all day long.
It really takes me out of the immersion of the game because all I see is that I am just a pair of floating limbs. I look down far too often when picking up items and the whole issue just seems weird and awkward to me.
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Just because it's a first person shooter that doesn't mean I can't take a look around and enjoy the visuals, jackass.