There are C#/XNA games on Steam. The developer, Phil Fish, claims it's built specifically for the "console/couch" experience on a Saturday morning (seemingly unaware that many people connect their PCs to televisions and sit on the couch).
Also, this has been an unusually long console hardware cycle resulting in there being a much bigger gap between PC and console graphics.
Is not about how many people connect their PCs to their televisions, but about how they enjoy playing their games. IMHO that does not justify the Microsoft exclusivity strategy.