This is an interesting question.
I think your question and the observation it asks aren’t just about lack of civility or closed-mindedness, but arise naturally from the general habits of thought that seem to especially permeate Internet discussions, but also much other modern Western culture (I think it stems from the duality in Judeo-Christian/Muslim polarized thinking). That is, we have very strong habits of thinking in patterns that are A versus B, even when (as is the case for most subjects) that model is grossly insufficient to model the truth. And yet often there is an underlying unconsidered assumption at one or many points that there is a true answer that involves either one argument or some conception of its opposite to be true. As many other people have also been thinking that way, we can find lots of agreement and evidence from other writers on the almost any subject.
It stands out to me especially in the frequent questions that are framed in ways that are not how I think about the subject at all. Yet often built into the question are the expected two counterpoints.
When two people both accept the premise that there is more or less one axis of the question, they’re unlikely to ever agree, because they don’t see that the truth isn’t really on the axis, and are looking at the topic from different perspectives. It’s like two people looking at two different optical illusions of the same situation. They’re both right from their own perspective, and much more could be said about it, but agreement could only be reached by abandoning one or more assumptions that are reducing a complex situation into different one-dimensional projections.