I mean, clearly they don't. Every other company is still installing Windows. Any of them could install and sell them with Linux, and save a few bucks in the process, but they dont. An official SteamOS release won't change that.
Isn't that because Microsoft either pays system integrators to only install Windows or threatens that they will stop providing relatively cheap Windows keys if they provide the option to start with Linux? I could have sworn I'd heard that somewhere.
Microsoft make big buck by everyone and their dog having Windows PC and paying for O365. Especially corpos, that also pay for Teams, skype, azure, etc.