The obvious reason that you have not seen a locked up alternator is that you are not in the trade.
Bearing failure is a common form of alternator failure and it is fixed simply by replacing the bearing, if a skilled technician listens to the motor in the last few thousand miles before it locks up. Picking up the noise in bearings of any of the accesories of a modern motor (including belt tentioners and belt idlers) is one of the reasons for paying an experienced tech to service your car.
Cars I service don't lock up alternators, cars I repair do.
Bosch Master, ASE Master, L1
33 years MB technician