Because society forces a set of rules upon us all that we must adhere to in order to maintain a standard that allows us all to walk around without killing each other and exchanging fake pleasantries in order to avoid conflicts.

Online there is no physical interaction so it allows people to be, think and say what they really feel with no immediate physical negatives. Most folks would be surprised just how negative we all truly are at heart if there was a way to live in the outside world without the filter on.

You'd be blown away by how many people are homophobic, racist, abusive and predatory but show no signs because a physical society has consequences for that mindset.