You know how there is this trope of people with little power in their lives abusing service industry workers? I feel like the behavior you describe is kind of like that. People go online and be way ruder than they would be irl, because they can. They can't empathize with the person on the other side, or they don't care and are happy to have an outlet without consequences.
I mostly assume replies are not actually to me, they are just people who want to hear themselves talk. Makes it a bit less frustrating.