That is because people do not test them correctly.
Anyway almost is not never.
I think that most electical failure is either caused by rough handling or incorrect replacing of parts rather than breakdown of insulation due to age.
But when do we do ESTs? After delivery by a courier, being dropped by the user - rough handling - or after we have taken them apart.
If we are doing the test when they are most likely to fail we should be doing them right. As for a yearly tests - well if you have the time to spare and nothing better to do......
Robert
