Think of it this way. Try closing your left eye and look left with only the right eye. Can you see things? Sure, you do. But can you really 'see/focus' on them with any accuracy?
In boxing, when for some reason one of the your eyes are out of function(like blood getting into that eye..), the opponent tries to slip into the side of the bad eye. You can see his body mass move, and detect large movements with peripheral vision. However you're not gonna see a fast punch coming in when he keeps moving to the closed eye. You can't focus on the point with just your right eye.
So, is someone gonna be able to see a friendly wingman looming around his 5 O'c from distances like 30 yards, when he looks left over his shoulder and 'sees' towards direction of the right horiz stab? - Probably.
Is he gonna be able to see a hostile enemy coming in at his 5 O'c from distances like two thousand yards, when he looks left over his shoulder and 'sees' towards direction of the right horiz stab? - I think not.
.........
The "the human neck and brain/eye coordination" is over-hyped.
- Your eyes may see something, but if its in an angle difficult to focus, your brain's confusion is sometimes very difficult to handle. It actively throws away the info registered that cannot be translated - people refer to this phenomenon in more simple term as 'slipped concentration'.
- Your eyes may see something, but if it is already focused in on something, the rest of the peripheral info(the 'periphery' is increased when the eye focuses on a single point) are either disregarded or pushed away from the priorities where your brain reminds you to 'update the info' - people call this 'target fixation'
- If your brain realizes that looking at a certain angle will not be efficient, it immediately compells you to turn the neck the other way around, so it has clear vision.
A vision which is not clear, which cannot be grasped directly via image, due to various reasons causes uncomfortable feelings, and sometimes can even cause pain. Such unclear info is often ignored or not realized - for the sake of your mind.
.......
And you're saying that a pilot can just look one side of the shoulder and immeditely get a clear picture of the things going on behind him?