Because of user karma. Even a fake incentive to say things that everyone likes beyond normal social pressure creates a bunch of people who eagerly say inane shit to get moar doots.
Think of it like this. When humans talk to humans, is any joke ever obligatory? It's "that's what she said" any time anything vaguely prurient gets mentioned.
Now imagine if they said "I'm obligated to tell you that's what she said." Do you see how they've added a tragic undercut to a comment that already wasn't funny?
I used to use /s all the time over at Reddit - especially in political discussions. If I posted sarcastically "advocating" for something, I didn't want people to misread the post and think I seriously supported that thing.
Normally, I could trust that people would pick up on the sarcasm, but it's hard over text and there were people actually advocating for the horrible stuff. I didn't want to be mistaken for one of them, so I'd add a /s. It definitely ruined the joke, but I'd rather do that than have someone think I was racist/sexist/bigoted/etc.