I suspect many of us might irritate other GN members with our style of posts. I'm probably both an irritant - and at the same time, one of the irritated.
I do think you are being unreasonable, because social media invites all-comers and people have their own individual style of posting and responding.
You have demanded to know "why on earth do you do it?" I have occasionally used this technique but mostly not. Why do I do it? Well - I haven't thought too deeply about it but I use this simple word to emphasise agreement, because it's in vogue and understood. And I suspect most posters do it for the same reason. I don't believe they do it to annoy or offend, it's just a typical social media tool.
You say it makes your blood boil. I don't know what to say about that.
However, as Doodledog says, you are risking making posters feel self-conscious about their posting style by suggesting it is a stupid and pointless thing to do.