Online it can become a competitive thing. They still want to “win” the argument even if the light comes on and they realize they have incorrect or incomplete information and the other side has made better arguments with better evidence. I suspect most people fall into this trap at some point in online forums. I definitely have. Guilty as charged.
And not to excuse this behavior, but part of it comes from poor sportsmanship and lack of grace from damned near everyone vis a vis Twitter/X/Facebook/reddit. People who “win” a competition like this are quick to gloat on how they “owned” the other person. Worse than this, trying to be reasonable and open-minded in these spaces often comes across as weakness.
There is a small movement of folks who call for radical empathy. This is where you do your level best to make a good faith effort to fully understand and see the merits of the other side’s arguments even if you don’t agree. That way lies learning and growth.