I don't buy it.
I think there is simple statistical/neurological explanation.
Part 1:
We are exposed to much more information and interaction that we have evolved for. Our brains have very biased/impractical approach to understanding the world: if you hear about something happening many times in large number then it automatically gives large weight to it and treats it as normal/prevalent/dangerous etc.
This is also what somebody might mean when they say "a lie repeated frequently enough becomes truth". That's how our brains are built.
Part 2:
We tend to notice things that are out of ordinary more than normal. Nobody spends time revisiting "normal" comments, but people will notice and spend their focus disproportionately more on mean behavior.
Part 3:
Internet amplifies things, but mostly things we focus on. This means an extremely mean comment will tend to be amplified more and get more visibility than a perfectly normal comment.
Part 4:
Even if 1 percent of 1 percent of people write an extremely mean comment just once that is still deluge of meanness that your biased brain will understand as "mean" being frequent behavior on the internet, something that is done by many people and probably frequently.