Because some are so insecure they need to have the last word, maybe due to the fear of loss of sleep unless they can get one last one-up in.
Various reasons for this age old conundrum, influence of: upbringing, schooling, religion, political views, peer acceptance, misunderstanding of a read, false information, miseducation or the lack of it, etc…
It's the internet, I’m just typing out into digital land and not bothered by the feedback, it’s quite interesting at times but nothing new as far as groups of people engaging in discourse, some calm and some not so calm.