14 years ago the answer was Oracle Linux, based on what I was told by my boss who left us to take a job at Apple
not sure what flavor / distro they use nowadays but almost certainly Linux and almost certainly one with an Enterprise Support agreement.
Because they have the equipment and are always looking for an excuse to use it to justify the cost and training. And because the more they use it, the sooner they can get more of it.
It makes zero sense to a normal person, nor does their justification for needing the equipment in the first place, but once they have it they'll use it any chance they get.