Because it's the Internet and it can be very difficult to tell what some people's intentions are. They could be "good trolls" and that's why their intentions aren't clear...because they're so sly about their malice, and this often happens I've noticed with Ts, very rational IxTx types who are good at covering their tracks, because they didn't use certain key words, then they can't get in trouble. Even though they know they were trolling you, you can't prove it. I am of the opinion that IxTx types are the most insidious trolls, though xxFP or ExTP trolls may bring more fun or laughter or obvious drama for which they are sometimes immediately banned.
But you really can't tell what people's intentions are. Like seriously one day I was making fun of myself for reading Elfboy's thread title wrong, and he totally went off, thinking I was personally attacking him or calling him a liar, and I was like
and was truly utterly surprised. And I actually like Elfboy so I felt bad that he thought I was viciously attacking him, because I wasn't, I have taken playful jabs at him in the past, but they were playful and not ...evil. I don't like to REALLY hurt people.
However, there are people who do; those people do like to hurt others, it amuses them to get a rise out of anyone they can, or they have a bone to pick with you and are trying to get you riled up or even in trouble or banned for arguing with them. These trolls are insidious but I can usually spot them because they tend to never apologize or say they were joking, and they tend to personally insult you repeatedly, sometimes arriving in a thread *specifically* to focus on you and not the topic of the argument. Gish does this to me a lot, and I'm not sure why I haven't reported him more, honestly.