Honestly, I think a lot of it stems from the fact that it's incredibly easy to just not resolve issues. If you don't like someone, you can just avoid them and complain to your friends about them. You'll never really end up in a situation where your actions have consequences, as new GM's put up new RP's every day and you can just make a new account. Grudges are easier to hold and often times when you complain about someone somewhere they can't see, you'll exaggerate and blow things out of proportion because you can. 

Yes, societies have evolved and we know that people can get past differences - but there's no real need on RPG to do that. We aren't depending on each other for survival. If we hate each other, we can cuss each other out and act as toxic as we want and worst case scenario, one or both of you leaves RPG. You might make new accounts or you might go somewhere else. The anonymity of a username and the fact that any consequences are just so feeble makes it easy to be assholes to each other. 

On a more positive note... I think for integrating new RPers into a group, it's key to make sure to keep them involved. I try to tag new users in my discord general chats, ask them how they're doing, what their hobbies and interests are. You can't force people to become friends, but I think that just actively making sure they're included is the best way to go about things.