The voting behaviour around here is .. interesting.
As for the /s, that's because of how humans communicate. Most of our context cues are subliminal, facial expression, tone of voice, body language, eye movement, etc. These account for the bulk of the actual message.
In text based communication like this, those cues are missing entirely. We use emojis and smiley face brackets to give a clue to what is going on.
Then there's the language barrier, people come from different countries with different languages, but also different cultural ideas of what's funny and what isn't. You can tell when you start looking into swearing for example. The Dutch hurl contagious diseases at each other, the Italians diss each others mother, Australians use body parts.
Sarcasm is a special form of interaction, straddling truth and disbelief in some way. It's not universally recognised in the same way since it often triggers off cultural beliefs which vary across the world.
So, to bring all that together, the simple "/s" shows the reader that sarcasm is intended in a more or less universal way. Of course that too has a cultural impact, but that's a rabbit hole I'm not going down today.