There are a couple of reasons. One is that they involve processes and kit that are seldom used. When they do come to be used they haven't had the shakedown that daily use involves so are likely to be more fragile and, in the case of equipment, possibly expired of old age. Of course that leads to a situation where regular testing is avoided due to fear which just makes the situation worse.
The other is that they only come into play in extreme situations which might be so extreme as to exceed their capabilities, e.g. a lightning strike that took out all the thyristors in the building UPS leaving the non-UPS power unaffected.
Biting the hand that feeds IT
