Consider the following: Even before this information became public, it was stated that Valve has the game in “bits and pieces” right now. This isn’t like having an alpha – there is no game to speak of. Putting all that together takes time, and with the amount of resources being devoted to Valve’s Steam Machines, Steam Controller and SteamOS, it looks like we’ll need to wait even longer than ever before Half Life 3 releases.
It could be that Valve is testing the waters with the Steam Machines to see if it would be viable to have the game as an exclusive for the same. Though it would obviously arrive on PC, how many units could Valve sell if their long-awaited AAA title was a timed exclusive for the Steam Box?
This is all conjecture for now, and it may be another two or three years before we hear any news regarding Half Life 3. Then again, Valve could also just show up one fine day at E3 and showcase the game, as it did with Half Life 2. Wait and watch, as always.