Honestly, at this point I don't know WHY the devs don't tell anyone what they're working on. It would save THEM time if what they're working on is not something the managers want, and it would save US time from having to deal with bad updates every couple months. If you're that concerned with making sure every update is a "special surprise", then you either need to re-evaluate your priorities, or look for a new job.

And in case an admin is reading this and thinking "we can't share what we're working on in case someone steals it": If you were that worried about other developers stealing your ideas, you'd be more concerned with improving the actual quality of the game. I'm not buying that excuse.