Not because I think I'm right (I think I'm aware of my flaws), but because I realize I don't feel like arguing about it, maybe my reply is closer to a belief of mine I've no wish to defend, and I suspect any snarky replies might incite a bad back-and-forth that never ends well online, bet it HN or elsewhere. So having gotten the reply out of my system, I delete it before anyone has a chance to reply.
Somehow my brain gets tricked into thinking I've replied, and allows me to drop the whole conversation in peace. It's a neat trick.
If you're right, or at least, not making a complete ass of yourself, chances are someone else is going to come along and argue back for you. And besides the benefit that someone else will likely explain your position in a slightly different way, and the multiple POVs might combine more effectively, having a half dozen people explain to someone why they're wrong is a lot more convincing to random bystanders than two morons replying "nuh-uh" to each other a few dozen times.
And if no one jumps in to defend you, it's a pretty good signal to step back and re-read everything and have a good think before you have another go at it, at least to make sure you have something to add beyond what you have already said, even if you think you're still right.