If “trying to find common ground and building from there” is normal human behavior, then normal behavior is in the minority these days. I haven’t had an in-person conversation with someone that disagrees with me that is even remotely attempting to find common ground in a very long time. It’s definitely not typical in my experience.

Argumentative accounts that personally attack and downvote, even though there is absolutely zero reason - there wasn't even any disagreement on the main points, just anger on minor details.

That's been here for a while. When I became a mod of c/politics some users stalked my profile to use it as proof my me being a terrible choice. They're evidence was me not having a degree in political science, disagreeing with them on some issues, and having a complicated relationship with my mom. I said simply check the modlogs, and there haven't been any complaints since.

Normal human behaviour is to try and find common ground and build from there.

This is wildly untrue. Even outside the confines of the Internet. People tend to circle the wagons when their beliefs are threatened, rather than try and find common ground. Which is why the way we debate ( and I'm debating now) is so ineffectual. You need to guide the person to come to the conclusion themselves (which is why the Socratic method is so widely respected), not tell them they are wrong and you are right and here's why. It's a low success method.

On the Internet, it's even less true. Now I'm just a dehumanized bunch of words, not even an individual, so your mind is rushing to try and figure out to categorize me so you can make assumptions about me. This just compounds the above issues.

Your argument seems to rest on the claim that this is atypical human behavior. This is a false assumption, and thus conclusions based on it are faulty.