Microsoft doesn't want to rely on licensed software every time they install their programs either. Again, Valve taking a queue from MS. And that's fine BTW, the whole industry follows MS.
Moreover the real issue, the difference in computing cost between running Win10 with all the unnecessary boost vs Linux is massive. Had they used Windows it would've costed more to be able to run less.
As to being reliant on Windows, that's been their standard most of their history. Steam was Windows based. If Windows were to go ahead with making a stripped down Windows OS that was specific to gaming, such as the one demoed in a code jam earlier this year, you can bet steam would be selling that version of Windows direct from their store, and likely have a easy tool ready to use to install it to your deck. They would probably offer it as an installation option too. Why not? There's no good reason they shouldn't. The whole verified question goes out the window. That's huge. But again, MS controls that situation, not Valve. They're still reliant on MS in major ways.