The answer is very simple: attention (aka social reward).

They see others behave, see the results, and mimic it hoping to get the same in return. Sometimes people just need to be liked, loved, and appreciated and their immediate environment isn't giving them that. So many cling to the internet as a lifeline for emotional support. It's a sad state of affairs, but people are just trying to get by. Unfortunately, some of the behaviour they end up mimicking is much less polite or favorable, but it got a result and so it gets copied.

Take care of those close to you so they don't need to resort to this just to get through their day. Moreover, take care of yourself.