Just kinda ruins the game, which makes people not want to play.
People get really aggressive when they built their identity around “being good at x”. So if you’re their teammate and you make a decision that they think is wrong, they see you as losing the game for them (even if you think it was right).
Same as sports, but you’re generally playing with random anonymous people online that changes every game, so there’s no incentive for long-term teamwork or relationship building.
Imagine playing casual pick-up basketball in the park on a team of 5, and you try one play that doesn’t quite work, and your teammates first reaction is to go “Wow you must really suck at basketball to try that!” Or: “Go home and quit, dumbass.” You’d probably feel a little bad, but also just never want to play with that group again. Now imagine 70% of all basketball games you try to play have people yelling at you like that. You’d probably just quit trying to play basketball.
There are also some really friendly people in these games (I always tried to be someone who shuts down the mean people), but the problem is that eventually the friendly people just don’t want to play your game at all anymore because of so many negative interactions.
They obviously want to grow the number of players, so having a reputation of a horribly toxic community makes people avoid the game, which is bad for profits (and the non-toxic players).
Not sure how much this change will help, because the bigger source of toxicity is a player’s own team. But it still could be a good step towards trying to fix their image.