Besides the possibility of getting "stuck" behind a pin, as someone else pointed out.. think about this.
For many tighter locks, I have to tip my hook to get under the pins. If the angle of my hook is, say, 40 degrees, I would have to tip it about 40 degrees to get the tip flat against the bottom of the keyway. If it were possible to tip it that far. To get a 90 degree hook's tip flat against the bottom of the keyway... well, you can figure that out for yourself.
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