The answer is because the risk of not cupping will create the potential for swatting (the hands flipping the club horizontally through impact). Swatting is a big hidden power leak. You feel like you are creating clubhead speed and maybe you are, but there are many other bad factors you are introducing:
1) Steepening the angle of attck - might be ok for wedges, but the ball flight is now high and just about every shot likes a low boring tragectory. If you want hight, adjust the ball position
2) Increasing the error % of a square face - Because your hands are the ones causing the face to square up now, they are prone to bigger errors of hooks and slices.

Cupping the right hand ensures that the right arm stays connected to the hip (no excessive out-in path) and

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I'm a golfaholic, no question about that. Counseling wouldn't help me. They'd have to put me in prison, and then I'd talk the warden into building a hole or two and teach him how to play. ~Lee Trevino