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Sarcasm usually requires something extremely outlandish to be said, or relies solely on the inflections of their voice. When there is no context to work off of, it's almost impossible to tell if someone is being sincere or sarcastic. This is known as Poe's Law. There are some news outlets that I know to be satirical, such as The Onion. If I was unaware that The Onion was a satire newspaper, I would either take what they said seriously, or believe that they are mentally challenged.
We need context as well as other indicators of such sarcasm, such as inflections and body language, two things that cannot be exhibited through text. Sure, we could use formatting, but that's just too much work, even if it's just a few special characters such as asterisks or tildes, a /s is just so much easier to do.