I think it'd because the Trackmania arcade mode doesn't really let you affect anyone else. Everyone in the server is obviously trying to go as fast as possible,and to a degree also get as high up on the session leaderboard as possible, but you can't ruin each others laps. And while finishing high on the session leaderboard is nice, the overall goal for everyone is to get a good enough time on the map in general. To beat your own PB, to get all medals. So essentially the real opponent is your past self. This leads people to get quite cooperative. Having discussions about how to tackle certain parts of the track, congretulating each other with setting a new PB, etc.

Playing against random people and especially with random people in other competitive games seems to generally get toxic. People blaming you for ruining their game, people getting mad. To me it's very stressful. Even if I know that it shouldn't affect me, and I'm never going to meet them again, it still does affect me negatively