Steam only tells you if the game is released, but not if all of a games content has shipped (for instance post launch dlc, seasons etc). Its an imperfect system and it definitely isn't something everyone cares about to be fair.
To me, it seems like there will probably be more units of DLC coming the more of a success the game is proving so far, and that it could keep coming pretty much indefinitely. So if you viewed such a game as "not done" you'd sit out a lot of widely loved new games "waiting for them to be done" for a long time.