At least part of the driver for this is to ensure that the servers are secure. Apple has long suspected that servers it ordered from the traditional supply chain were intercepted during shipping, with additional chips and firmware added to them by unknown third parties in order to make them vulnerable to infiltration, according to a person familiar with the matter. At one point, Apple even assigned people to take photographs of motherboards and annotate the function of each chip, explaining why it was supposed to be there. Building its own servers with motherboards it designed would be the most surefire way for Apple to prevent unauthorized snooping via extra chips.
I can’t say this is terribly surprising. There’s been reports of this sort of thing happening to Cisco hardware among others.