yeah if you don't have access to the files it would be extremely difficult, the only way you could realistically "pirate" a game that is only available via such a streaming platform you be either using stolen credit cards/game codes or maybe using tunneling software to share games with multiple people (assuming the platform has limits on the amount of devices allowed, and even then any accounts doing that could be banned if they share their account too much)....or some major hacking work to break into their servers and obtain a copy of the game from there, definitely not the most realistic things to do
movies etc can be pirated easily as its just a audio/visual medium that can always be copied no matter how hard they try to DRM it, short of some very strictly guarded exclusive viewings with xray machines etc to stop people taking cameras in
but yeah I think steaming exclusive games are going to be a good while off yet as it would cripple the potential sales to only the most determined people with great internet connections