Because there's a fine fine line between sarcasm and cruelty. There are ways to portray sarcasm which will prevent someone from knowing it's cruel...a smile, a wink of the eye, a twinkle in their eye, and the tone of voice. People who are being cruel and "saying" they're just being sarcastic know exactly what they're really doing.
Although many may think so, it's not always about IQ. It may have to do with the personality of the sarcastic person. If it's either a very monotone person or someone you don't know on a deep enough level, it makes it a little more difficult to digest. Then you've just got your garden variety whack jobs...
there are many ways of understanding literally and sarcastically are the two most common some peoples brains are "hardwired" to understand in a more literal way as opposed to people who can easily detect sarcasm. also it is the tone of voice that the sarcastic person uses that throws the other person off because at times the sarcastic tone can be easily mistaken for a more literal tone there is a certain manner of speaking and understanding that is needed for sarcasm to be effective
Some people just think is rude. I don't and can be as sarcastic as they come. Though when you are talking about something being written there is no way to convey connotation and can come off as rude, unless you make a note of some kind, such as an eye roll.
Because its usually not nice! Sarcasm is backwards thinking mockery.... Guessing you really have to think outside the norm and be pretty secure to accept sarcasm. I usually only use sarcasm when I'm angry at someone, so it doesn't come from a good place.