Because the failure mechanism for rolling element bearings isn't pure wear. The bearings fail via a fatigue mechanism where the crack begins at a sub-surface defect.
The initial defects are too small to be detected, and are distributed randomly through the bearing material.
If you get an individual bearing with a slightly larger defect, or a defect which is aligned normally to the principal stress direction, or located exactly at the location where the stress is highest (again, this is below the surface), then, you'll get a bearing with a very short life.
This is included in bearing design, sizing, and lifing calculations, where the so-called L10 life is used - the life where 10% of the bearings will have failed.
You can't design a mechanism with rolling element bearings to be absolutely free from bearing failures, and you have to accept that if you make thousands of items, you WILL get some bearing warranty returns.
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Volkswagen Polo III - whining noise 
Volkswagen Polo III - whining noise 