Every copy has to be hand made by routing bits around the copper highway ar ludicrous speeds, and rearrange them manually to form what is called "a game".
Firstly, how dare you! Secondly, unity is made from a limited resource, which is whale balls. For every download of unity, a whale loses one of its balls. Think of the whales!
So if Microsoft published a Unity developed game on Windows, Microsoft could easily charge a $0.20 free to the unity team for installing the Unity Runtime on their OS.
Not being completely serious there. Honestly thought, did the CEO not realize if they start doing this, what's to stop another company from doing that to them. Things like mp3, where developers need to pay a license for, could then be charged in a similar fashion for each install.