I've almost never changed my mind in an online argument but I do regularly offline. Why is that?

I think it's because online nobody acts in good faith. There is no connection and trust.

I had a customer once who would just absolutely berate people over email for the tiniest thing. Totally unbearable and unreasonable. So, whenever he would go off, I'd tell him 'look, I'll be in the area [this afternoon|later|whenever]. You going to be around if I stop by?' Any conversation with him that could be deflected to an in-person discussion could be peacefully resolved in short order. Trying to convince him of anything over phone or email was an exercise in frustration control.

I heard somebody say at a conference one time, talking about how much more productive in-person meetings are in reaching agreement, "there's a lot of bandwidth in a room". I think there's a lot of truth to that.

0 - ironically, this was at a ISP network engineering conference

I think you can have two people who, both acting in good faith, can completely lose it over textual communication. Even a phone call can make the same discussion ten times easier.

I have changed my mind on things in online discussions. Usually in the form of them saying something I disagree with, me trying to articulate why they're wrong or searching for evidence that they're wrong, and then coming up short or finding information I wasn't previously aware of. I find this is more likely to happen online than IRL since you don't really interrupt IRL conversations to go spend 3 hours looking for sources, though I do also get what you mean about a lot of online comments being so blatantly hostile that you just don't engage.

Are you saying your comment here is in bad faith? ;)