Comments would be made for the benefit of the peanut gallery. People want to hear incisive comments, zingers, cleverness, impassioned rants, etc etc. This is what imaginary internet points rewards.

With a one on one conversation, you're being judged by one person and one person only. You can express ignorance, honest but controversial beliefs you may not wish to commit to the internet for all eternity, and so on.

Public forums with imaginary internet points are a skinner box. HN may be better than a lot of others but it's still the same beast.