OBollocks is right, and the competition is not limited to game ideas, it also encompasses the game talent--the devs want to retain the people they have working for them without having to treat them like human beings or up their salary. So the less info anyone shares about anything anyone is doing, the more of a culture of fear and suspicion is created, leading most humans to feel like they're safer staying where they are. Despite regular all-nighters, utterly insane/spoiled management, and zero public recognition for their abilities.Part of it is instinctive, part of it is because just about everyone else does it, and part of it is because you can't copyright your game mechanics, and even patents for certain aspects can be granted but still lead to long, expensive lawsuits. There is vicious competition that, while often carried out politely, is still savage.This might sound totally naive, but why isn't everybody basically open about everything?
I know this not from personal experience (thankfully), but from the anecdotal evidence told by friends and acquaintances who work for some of the bigger devs in this area.