Why can we share with our friends, foes, strangers, and Humankind on forums many things which we would not discuss with anyone offline?
As Capt Nice said, we're anonymous. There is no "threat" sharing whatever you want online (for the most part). You don't have to worry about damaging relationships, offending someone you may care about, violence, etc. None of us will ever meet each other in real life so it's basically a free for all. That's also what makes the internet such a toxic place though. The overwhelmingly vast majority of people would never actually say the things they say online in real life due to real life consequences. Online anonymity emboldens folks like nothing the world has ever seen before which can sometimes have pretty big consequences if you're somebody like my buddy who has a really big mouth during online video games who quickly realized that talking **** to people in real life doesn't work quite the same way lol....
Also I can discuss topics which people offline find offensive. Even my parents. G-d bless my parents for supporting me!