Who rips the $1,000 processors and $200 voltage control modules out of their servers to upgrade to $2,000 processors and $250 voltage control modules, then re-plans their entire infrastructure and possibly code back around that?

Symmetry between servers has a value.

The main board, raid controller, network, and usually the storage is going to be planned out meticulously ahead of time based on the maximum load the server is going to see during it's lifetime. Often, Processor and Memory come down to "If we needed X Feature and we didn't have it" or "If we took the server down for 1 hour to upgrade it, how expensive is that?".