Actually that's a good question. Perhaps an answer is that they are looking for still-supported firmware or needed WOL as such a core feature and perhaps apps couldn't just run on just Linux, but it is still a good question: why run a closed-source (some might call spyware) operating system on top of libreboot, the opposite.

Although it is indeed a weird mix of going all the way of using a FOSS firmware with the goal to remove all non-FOSS proprietary software blobs on the boot-path and then include them anyways via Windows, but that is of course up to the consumer. The right to use your device anyway you wish, the ultimate expression of libre.

Why would someone run their proprietary server software on top of Linux?