It's a pretty reasonable thing to be concerned about, but I think it's so that the gun you're holding doesn't block the entire screen. If it was where it was, yet the reticle was in the middle of the screen, it would be constantly firing upwards slightly, and MC would have to adjust the angle for range, rather than firing straight. Also, I agree it cuts off the lower part, but that means it's easier to spot enemies if you are on higher ground, which is correct, and allows you to see banshees and Phantoms more easily. Whatever the reason, it's there on purpose.