Some of the main reasons would be:
- As you say, the idea may be stolen. But that actually counts for less than people think. Take a look at this thread. It's the execution that has real value.
- (definitely true in my case) The person may still be a newbie and/or may not have fleshed out the idea enough to defend it or make convincing arguments about its value. So he/she might be reticient about publicizing it.
As for the real world, people are definitely more open. I was at the Silicon Valley Co-Founders Meetup last Thursday (hosted by Alain Raynaud ). There was a LOT of frank and open discussion about ideas, because the raison d'etre of the Meetup is to be able to locate Co-Founders for your startup.