I feel like it's a combination of anonymity making you invincible: If the other person attacks you with the infos you've given them, you just close the chat window and continue with your life.

...and anonymity stopping others from helping you: If you open up to someone, there's an implicit expectation that they should try to help you, if they're not an asshole. But many people don't actually want help, because they don't want to be a burden. They just want someone to listen.
So, you either open up to an asshole (which is how many, traditionally mostly male friendships function), or you open up to some trapped behind a glass screen.