My take is that written communication is hard, unless a) you know each other really well, e.g. messaging friends, or b) you write carefully and with enough detail to help the other person understand fully your position, and they bother reading with the same care.

When you read an essay or article it of often begins by setting out the problem, giving some context and even defining their priorities and approach, before they make a claim or argument. They spend time addressing the obvious criticisms of their argument, and ideally admiting weak spots, and maybe even empathising with why someone might reject their position. This means that when you read an article like that, even if argues against something important to you, you don't feel attacked. It's calm, general reasoning, and obviously not a personal a attack on you as an individual.

But if you post an picture of the secondhand car you've saved for two years to afford, and the first comment is "fuck cars, they're killing the planet" it's easy to feel like it's a personal and it's aggressive. Or if you write a pretty reasonable but contraversial opinion, people might not have the time or will to break it down and explain why it's wrong, but they don't want other people to read it and think it's okay, so they down vote and comment a quick "what is this shit ?"