I obviously can’t speak for everyone but my assumption is that there is such fundamental disagreement that for many, even saying “agree to disagree” with certain people would be a betrayal of their own values.

Someone I know put it pretty well when talking about the idea of how accepting their workplace should be of people’s different political values. They said “If someone doesn’t think I should have human rights, I don’t want to work with them.” which I think makes the point pretty well. See also: paradox of tolerance.