I wonder if it's because we don't have the flood of repeat content (reblogs, retweets, etc.) to hide behind, combined with the fact that you own a personal space (unlike the chatroom-style things like IRC, Discord, etc.). On the other hand, LJlikes are also home to the anonmeme format, which for all its potential meanness also cannot be described as anything but informal/off-the-cuff.

Anonmeme format is also inherently social, though -- it's not the same as Twitterish venting-into-the-void. It's been literactual decades since I last participated in any, but IIRC if you wanted to vent without the assumption that you were inviting responses, you either found a vent thread or created one, rather than venting directly in a top-level thread.

I do wonder whether anyone has used the comments on their own page for a similar purpose, though. I feel like maybe I've seen roleplayers use comments? Maybe? I might be making things up or remembering outdated things...