Earlier quoted context omitted.
Because VR is just another way to sell games. They want you to know you can play your steam games on everything and that is their primary priority, and since things like games from steam can run on your Oculus (or whatever) they can sell you on a thousand experiences instead of one original sale.
Until Facebook decides that it wants a cut of all the sales on Oculus and locks down the platform, hence Valve's concern. There's only one App store on iOS vs Android. Valve's main bargaining chip would be not allowing their games to run on Oculus. The recent push back on Oculus exclusives and Valve willing to float cash to developers means they are well aware of their situation.
maybe I misunderstand but Adroid is targeted by many app shops.