My theory is that it's that on many of these sites there's no easy way to comment, like or otherwise publicly interact. Sure you could try and email the person but that takes effort and you have to talk directly to them, not to a crowd.
When you don't have to worry about a mob of negativity, you can write far more freely.
Yup, social media is a one-to-many relationship, and sometimes the messy many-to-many relationship. Web 1.0 sites have more have a one-to-one posture (unless they become exceedingly popular at least)
When you don't have to worry about a mob of negativity, you can write far more freely.