Why do rotors/flywheels have tapered shafts?

The rotor on my '79 kz750 fits onto a smooth tapered shaft. It does not freely spin either. Its rotation is fixed to that of the crank shaft rotation. Is there a reason for the tapered shaft? Why not fit it onto a toothed shaft like the clutch hub?

Machine tools have tapered shaft too. Most drill presses have a taper on the quill to fit the Jacobs chuck, usually a MT 2 or 3. Lathes have again, tapered shafts. If you really want a tight fit, get some lapping compound, and turn the rotor on the shaft by hand, cleaning it off once in a while to check the fit.