If they're building the game for Windows (so not purely console) it's a lot easier to be able to run the game on the development machine if that machine is runn

If they're building the game for Windows (so not purely console) it's a lot easier to be able to run the game on the development machine if that machine is running Windows. If the dev machine runs Linux or OSX then either a reboot is needed (no debugging allowed) or a compatibility layer/emulator is needed (performance issues). Neither option works well. They could have a second test machine running Windows, but that's much more complicated, takes up more desk space, etc. It's generally much easier to write software on the target than it is to write on another OS/architecture/etc and cross-build.