https://www.shacknews.com/article/74262/missing-in-action-the-curious-no-shows-of-e3-2012
Valve, nor any of their games, not even mentioned.
Why should they? They were there, but only showed off CS GO. Their games aren’t in that list, because Valve said that they weren’t going to show any other games on this E3.
The majority of those games are in that list, because the devs of them didn’t even mention them for a long time neither did they say anything about them.
And I guess, because Valve has made a statement about HL3, recently.