The reason VR headsets don't work on Mac isn't because Valve decided to not support them... VR headsets typically plug into the HDMI port of "the latest greatest graphics card", which isn't really "a thing" on Apple computers (until maybe extremely recently--like less than two years ago--on the Mac Pro, and I am not even sure about that), and before you say it is because nVIDIA is simply not bothering to support Mac, I'm pretty sure it is because Apple doesn't really provide sufficient hardware ports or driver extension points for them (and then probably will get pissy about how they have lost control over color balance or something as their software probably only works "perfectly" with their hardware, etc.)