Modern culture (which is now Western or Western-influenced) is prone to assholeism for two reasons: 1. European and American culture separate images and symbols from reality. In their culture, words, pictures, oral expressions etc. are 'not real' and therefore cannot do damage. Its the responsibility of the insulted person to toughen up and learn that sticks and stones hurt bones but name calling does no harm. 2. Since the French revolution, liberal culture has embraced verbal and symbolic insults as a way of mocking or attacking your opponents e.g. jokes about Marion Antionette depicted raping her own son, political cartoons, Reddit atheist rantings or the way both American right wingers and progressives tend to portray each other as feeble, sexually degenerate, and stupid. So being an asshole to your enemies became a virtue, especially if you could position yourself as a heroic figure standing up to injustice and irrationality. The only cap on this is the idea of 'hate speech,' which is ambiguous leading to conflict about how it should be interpreted although both sides of the aisle more or less do this 'its only hate speech if someone I like gets insulted' thing. Anyway, all of this has lead to a culture where insulting and offending others is seen as a public good and even cool. The internet simply made doing it easier and low risk, less chance you'd get beat up if you call a guy a nigger online and if he gets mad you can always paint him as irrational. The internet also globalized these values so now they are pretty much everywhere.
In other human cultures (before modernity at least), people did not separate signs from reality in this way and a verbal insult was treated as similar to a physical assault. Since an attack had to be met with measured retribution, a strict culture of decorum developed. You can see this in Edo period Japan, where an insult could cause fights between samurai and violent young samurai would attack and even kill people when they felt offended. That's not saying there were no assholes back then or people didn't offend each other, but they weren't openly hurling slurs like you see online or in modern media generally. You can see similar processes play out in other cultures too.