I believe this happens due to two things:
1. These players avoid going to another server with teleport features and more prepared for 1v1, not because of the server, instead, due to that LWs is probably the most played place nowadays, and there is a little desire of playing with more people around than you and your friend. For instance, I enjoy variety when I play, some minutes driving around, talking and hanging around, teamplaying later and also dueling. If you join another server with 0 players just to duel, it might become boring after just minutes.
2. Many people might have the intention, myself included, to fight with their friends, but these friends who are against this, have an acquired culture of not-fighting you and when you do so, they make you look like you're the error, and they get angry - so simple, you choose to avoid fighting them to avoid a discussion.
I agree that fighting with friends is more entertainful than avoid shooting when they are in a different team, or picking the same skin. But it's a culture that is hard to remove.