But the process of pressing the game to disc is handled by a third party manufacturer. It's not the like the developers are cranking out copies of the game on disc in their spare time.
If you're going to put Steam's installer on the disc.. put the game's installer and files there too?
Burning what's available on disc and then applying a major update/patch on top of it can be more complicated than it sounds and it may well save them a lot of time to just release the game for digital download as a finished or nearly finished product. We've seen how fast they were cranking out major updates with the beta. Not to mention the added deadline of having a version to put on the disc finalized for the manufacturer. If they decided that for whatever reason it's better to just have a Steam installer on the disc and nothing else I have no doubt they have very good reasons for it.
