Why it is so simple to predict hard disk failure?

Failures can be seen from two standpoints: predictable and unpredictable. Unpredictable failures, such as power surges that can cause failure, occur quickly. Predictable failures are characterized by degradation of an attribute over time, before the disk drive fails. Many mechanical failures are typically considered predictable because of degradation in a disk drive's attributes such as head flying height, which would indicate a potential head crash. Certain electronic failures also show degradation before failing.