Why would Apple ever invite Valve to potentially do the same to them?
I actually see it as the reverse. Valve might be going for the whole pie and want to carve out a niche for their Steam Box. Inviting Apple to the party might detract from that effort. Or at the very least distract from their main focus.
They don't need Apple for that. People who game already game elsewhere. Steam on Apple feels pointless. I wouldn't be surprised, if Valve will go for smartphones with their own at some point