It’s the algorithms. Except 4chan where that’s just how it do.
4chan was simply where everyone banned for extreme views everywhere else went because there were no rules or close to none.
Now as algorithms of social media sites have been chosen to dictate our daily news cycle, discussion topics and visibility of culture by engagement metrics alone, as are dominated by flashy, extremist content that appeals to our lowest instincts. Before humans curated content by trying to use human reasoning instead of relying on instinct alone. I think we have taken a step backwards in evolution culturally.
I don’t think it’s the internet. I remember 90s internet because I’m old apparently. Forums, mailing lists, blogs were the dominant form of internet media. And while trolls and bad people existed it wasn’t nearly as bad. People found together in interest groups and things mostly went fine. It was really when algorithm based browsing of content became the norm that things changed.
In my opinion the reason is that it is engagement driven instead of topic or content or interest group driven. And the strongest and most reliable form of engagement is anger and rage and overexaggeration. So we see a lot of that and people who are good at that become the loudest and most popular. It also incites anger towards others and thus splits societies. Fashists and populist thrive. We need to regulate social media ASAP.