Ego has been mentioned. Sunk cost fallacy plays a part. Combine those and people tend to over value the effort they've made to form their opinion, either through some form of information consumption or synthesis of known ideas/held opinions, while devaluing/doubting the existence other people's previous efforts. So, you often end up with two people who both think they're informed and assume the other person is just making up an opinion right now and failing to see their valuable insight.