Part of that may be a result of a possible lemma to Murphy's Law:
The probability of a more expensive/harder to get at component in a system protecting a cheaper part by failing first is inversely proportional to the ratio (least costly component/more costly component).
The probability of a more expensive/harder to get at component in a system protecting a cheaper part by failing first is inversely proportional to the ratio (least costly component/more costly component).
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