Another comment keyed onto a concrete example of why not, so I'll go with a presumptive reason: Some of those elements will be part of the major service window

Another comment keyed onto a concrete example of why not, so I'll go with a presumptive reason: Some of those elements will be part of the major service windows, and have expected operational and standby lifetimes. So if a component with two elements has a service window of 10 years, and each element contributes to meeting that service window, then you've bumped your major service window from 10 years to a significant factor less than that. e.g. the expected use profile might be: use element 1 for 6 years or 60% of service, switch to element 2 for 4 years, replace both during 10 year maintenance window. Interrupting that by bringing element 2 up reduces that window and contingency plans if the service window cannot be met. I don't know, and I'm just talking out my you-know-what.