There may be some more complex elements to this, but I don't think about this in a complex way. It's very simple for me. When you are face-to-face with a person, especially a person you know, there is a connection which forms through body language and other factors I can't identify which makes it very viscerally real that the person across from you is a human being who has their own unique experiences that have helped them to craft a particular worldview. It engenders at least some measure of respect, perhaps enough to give people the benefit of the doubt and to hear them out.

When you're online, you don't even know if you're talking to a person who is being sincere in their beliefs or a troll trying to get a rise out of you, or perhaps an advanced bot intended to drive discussion in a particular direction. There's no indication of the motivations, the sincerity, or even the reality of another human being when you interact online. I can't imagine how a conversation intended to be deep and nuanced couldn't be dehumanized by the Internet?