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.
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.
the fun of sarcasm is knowing enough to recognize it
if somebody is too out of the loop to recognize it and picks an argument, it's collateral damage