This is a good point, and it lead me to a realization: on reddit, there are two crowds that don’t get the joke. The first is the people the sarcasm makes fun of. The second is people on your side that just really love correcting people. Treating you like you’re serious is a chance to correct you and gain community approval for how “right” they are. They miss the sarcasm because they’re so excited to correct someone and gain community approval for it.
This isn’t a problem in real life: you know who you’re around, and you make sarcastic jokes when everyone around you knows your stance already. I can see why /s became a sort of necessary protection on reddit. We can hope to not have to protect ourselves from people like that here, and not need /s as a result.
https://reddark.untone.uk