[jacquesm]

That may be so, but in normal communications, such as speech it is tone of voice and possibly non-verbal hints that will indicate sarcasm. Online you don't have that and the 'collateral damage' can result in a lot of wasted bits from people reacting to the words rather than to the joke and this in turn can drown out the rest of the discussion.

[baobabKoodaa]

Adding a /s to a post is the internet equivalent of saying "just joking haha" after saying a joke. It ruins half of the fun.

[jacquesm]

https://en.wikipedia.org/wiki/Poe%27s_law

[baobabKoodaa]

I understand why you might want to add a sarcasm indicator to texts including sarcasm. I'm not arguing against that point. I'm trying to show the other side of the coin: why some other people would not want to add those sarcasm indicators.