Because it looks fucking creepy.
Here, try this:
Furrow your brow
Put on a big shit eating grin
Widen your eyelids, resisting the pressure from your brow
Then tilt your head downward a bit until the shadow from your brow darkens your eyes
Try lookin in a mirror and being impressed with how rapetastic you look. I bet you can't wait to try it out the next time you go raping, I sure as heck know I was!