Pretty sure at this point valve just does things for the love of the game. They don't need VR or handhelds and they certainly don't need their own games, but they do it anyway.
And I don't say that just cause 30% profit off their store is enough; Apple and Google both do that on mobile and they still stick their grubby hands on everything (their stores are probably a fraction of their profit anyway).
Nah, it's cause Valve doesn't have shareholders and doesn't seem interested in the myth of infinite growth. They just want to tinker; Portal was self reflecting parody, haha.