Except half the trick to deploying sarcasm is to use tone of voice, which you can’t do in a text-only format. /s is like a shortcut for that. To use a face-to-face example, it’s like saying something sarcastic with a straight face then cracking a smile to reveal you were joking all along.
Plus we’re on the internet, people have some terrible takes that totally seem like they should be sarcastic but just aren’t.
I guess I do get it though. /s does take some of the humor out of it, but it seems like more than a worthy tradeoff. People are just so hostile to it.
It’s a slash because it’s a shortened version of <sarcasm></sarcasm>, fake HTML tags that used to be used on forums.