Granular exclusivity would be antithetical to Valve's goal, which is to get Steam and its catalogue into as many hands as possible.
A succinct explanation:
Microsoft's share of a sale of a Valve game on Windows: 0
Valve's share of a sale of a Valve game on Windows: 100
Microsoft's share of a sale of a third party game on Steam on Windows: 0
Valve's share of a sale of a third party game on Steam on Windows: 30
Amount Valve makes on an OEM sale of a Windows license: 0
Microsoft's share of a sale of a Valve game on SteamOS: 0
Valve's share of a sale of a Valve game on SteamOS: 100
Microsoft's share of a sale of a third party game on Steam on SteamOS: 0
Valve's share of a sale of a third party game on Steam on SteamOS: 30
Amount Valve makes on an OEM sale of a SteamOS license: 0
How does it benefit Valve to punish Windows users? It doesn't. So they won't.