I think they also want to build up the hype on the game, to increase sales at the end.

There is a specific way in which game developers develop and release games. They GENERALLY don't give a date unless the actual programming is finished, or in the final stages of testing to ensure that they don't have to change and delay the date. It pisses people off more hen a date gets delayed than it does not having an exact date... therefore they don't plan the date unless they can 100% hit that date.

True. If they were to give out a release date, before the game was finished, and finished the game after the release date, problems could arise. Also, it might have also forced the developers to rush the game before the release date.