It’s a combination of factors. Language/cultural barriers are a big one. What is obviously sarcasm in your area of the world won’t necessarily make sense in another. Add in English as a second language, and it’s a crap shoot, even with an obvious joke.

The lack of tonal queues is also a problem. We communicate a lot via voice tone and body language. Without them, what is obvious to you can be read completely differently.

The last is the elephant in the room. Bigots dog whistling. I’ve seen too many “obviously sarcastic” jokes that are very much not sarcastic in a different group. When those people get called out, they fall back on “it’s just a joke”, the armour of arseholes the world over. By adding the /s preemptively, you rob them of cover to spread hate. It’s a variant of the nazi bar problem.