Because deforming the metal creates pressure that enhances the lock.

Once the metal has been deformed and thinned in places, it can't create the pressure it's intended to exert.

I will say that I was surprised to find a crush washer in that location. I'm used to seeing them only as seals in pressurized fluid systems (oil or coolant). But by definition, a crush washer is meant for a single use.

And I've never seen a crush washer so thin! Those are practically foil.