They spin at 5400 rpm for the older drives, 7200 rpm for the newer drives, Serial ATA are now around 10500 rpm.
Wear and tear on the bearings causes the platters to wobble putting more strain on the bearings which eventually leads to a noisy drive failure.
There are also the arms which read and write to the drives, they are constantly moving across the drive and they like any mechanical device can wear out.
The electronics that control the drives operation and the drives motor itself can burn out or develop faults too.
These are just some of the ways that a drive can suddenly fail