The core is empathy... you have to show that you genuinely accept and make space for the other persons idea and emotions, it's the mindset behind the strategy of active-listening. If you simply use the strategy of "so you're saying that" without genuinely trying to understand the other person, it comes off as straw-manning. Similarly, if you are telling someone else what their emotions are, instead of making space for their emotion, it comes off as judgmental.
I think something else you're getting at here is that this is something that's very hard to do with a stranger over the internet - the requisite body language, rapport, voice tone etc all get lost (and therefore more useful in most real-world cases of trying to change someone's mind)
Edit: one final thought on this is that active listening isn't much used in debate. It's much more useful for dialouge and rapport building than trying to convince someone with facts.
Edit: I didn't realize this before writing the post, but what I'm referring to is The Principle of Charity.
Story
I was confused about Node Modules, so I did a bunch of research to figure out how they work. Explaining things helps me to understand them, and I figured that others might benefit from my explanation, so I wrote a blog post about them. However, I'm inexperienced and still unsure of exactly what's going on, so I started the blog post off with a disclaimer:
My friend said that it's a bad idea to do that. He said:
I interpreted what he said literally and basically responded by saying:
This was stupid of me. He didn't mean "claim that you're 100% sure of what you've written". He didn't mean "pretend that you're way more confident in what you've written than what you really are". He meant, "I think that it comes across as you being less confident than you actually are. And so I think you should reword it to better communicate your confidence."
I shouldn't have interpreted what he said so literally. I should have thought about and responded to what I thought he meant to say. (Although, he also should have been more precise...)
Thesis
People often interpret and respond to statements literally. Instead of doing this, it's often useful to think about and respond to what the other person probably meant.
For example, "If I interpret what you said literally, then A. But you probably meant X, so B. If you meant Y, then C."
Depending on how confident you are in your interpretation, you should probably respond to a variety of possibilities. Like if you're < 80% sure that you know what they meant, you should probably respond to possibilities that have at least a 5% chance of being what they meant. I'm not sure whether 80 and 5 are the right numbers, but hopefully it communicates the point.
Why don't people do this?
I see two likely reasons:
Practical considerations