...because as the round is being chambered (assuming you aren't using a revolver, here), instead of a nice tapered ramp on the front of the round to slide it into place, you have the equivalent of a sheet-brass shovel, which could scratch its way into place, or just catch somewhere it isn't supposed to.
Just because a car key was made to go in a car, doesn't mean I want someone keying my side panels, ya know?