Two things. First off You aren't accounting for the fact the game isn't 100% played internally at Blizzard. Then they need to shift to Beta testing, which will need a couple months, maybe more depending. Then they need to release at a proper time. Also single player has to be finished, Cinematics, Voice Acting, Music, which who knows what level of completion that is.
It's also marketing. The game isn't coming out in January, no big game does that right after christmas. Either right before, or at least 3-4 months after, so earliest would be April, but more likely June-july, aka summer, which is what I said.
Just for Example in the last month the Jackal, Nomad and Archon were all redesigned, and the Nomad/Jackal got renamed. Not everything that "seems" complete is going to stay. If it was any other company than it probably would be a couple months but I highly doubt that.