Because some people are not arguing in good faith. Sometimes it’s best to call them out on what they’re doing so people learn to recognize the behavior.

This comment section is not the first time this topic has been thought about and discussed.

Well, I see it like this: A conversation can only ever be had under the assumption that all participants are acting in good faith. If that assumption breaks down then the conversation simply cannot continue. Once you are convinced beyond a reasonable doubt that your conversation partner does not want to share information with you or does not wish to convince you of their point of view but instead has some kind other motive that does not involve listening to and understanding your points, then there’s nothing else to do but to exit the conversation.

I can see your point that in the context of a public discussion board it may make some sense to consider a possible audience, but I also feel like this comes with such a rat’s tail of different problems that it’s probably not worth the effort. It provides a very perverse incentive for an argument to devolve into some kind of spectacle sport, where one ends up disregarding the conversational partner and instead is rewarded for focussing on an imaginary audience. I think that’s not desirable and it’s therefore best to treat every conversation as if it were private - if possible.