I wonder where all these reasonable adults are you seem to think are out there. They don’t seem to post much anywhere on the internet.
I have a table full of these reasonable people. I've had tables with reasonable teens, too. They don't post on social media to discuss RPGs, you're correct on that. I do recall seeing such behaviors in players... back when I was playing in Junior High School.
But, it still remains - there's a selection process here. If you don't select against a particular behavior, you'll probably see that behavior.