It could also be because it makes economic sense for Valve to make their software portable. Being tied to Direct3D, which only runs on Windows, is not in any wa

It could also be because it makes economic sense for Valve to make their software portable. Being tied to Direct3D, which only runs on Windows, is not in any way futureproof. I'm not following Valve closely, but they used to be working on the Steam Machine, and they have VR aspirations. I don't think they'd want to have to buy Windows licenses for their custom gaming hardware. I'm not sure how active those projects are right now, but regardless, if your game engines are tied to Windows, it locks down your future options, Microsoft has you by the balls. To maximize profits and future opportunities, you'd want to be able to write once and port easily to Mac, Linux, cellphones, consoles, VR headsets, etc.