Yeah, politics is the mindkiller.
Dan Kahan's research could lead to some interesting suggestions about why this might be. There are many questions we answer dispassionately about facts, but then there's this other series of questions about the world, where we really don't answer with reference to specific knowledge. They seem primarily to serve as litmus tests for measuring group identity.
As it happens, contradicting someone who is really just talking about their identity just makes everyone upset.
Certainly conversations can change some people's minds on some political topics. I've had a few rare 'conversion' experiences when talking with friends. But it's usually in really specific situations with certain approaches to open discussion that are just really hard, sometimes it takes multiple discussions, none of which seem that monumental, but have a cumulative effect. Both conversants have to rely on a lot of care and patience. All of that is made much more difficult by the impersonal and ephemeral nature of conversations on the internet.