I haven't worked in game Dev but I'd imagine that each person would want/require a copy of the game (dev-build, etc) that they could probably test their aspect

I haven't worked in game Dev but I'd imagine that each person would want/require a copy of the game (dev-build, etc) that they could probably test their aspect of development in, which is highly likely going to only run the platform of which they're developing for. Developing something for like Sony or Nintendo might be more OS agnostic in the developer's seat, but then it relies on the tools working on those OS's. Windows is never really a choice, it's more "I choose X, which only works reliably on windows"