Because they enjoy sharing info. Getting things "off their chests". Having an extra mind to think about the issue. Getting help from people, which they are only able to give inasmuch as they are clued in. Having other people relate to the experience, or running a sanity check against them. Difficulty in maintaining the illusion of the contrary; especially when you're lying about something that is essential to other people's understanding of your true self, that's basically life on hard mode. There are plenty of advantages, really, even in spite of incentives to keep some info secret.