>>712601738 (OP)Because in a matchmaking environment the social milieu trends towards being an absolute cesspit. Giving people an outlet for their instinctual need to castigate outsiders in the form of reporting directs some of that cesspit into "acceptable" forms of aggression, but that ultimately just leads to another flavor of cesspit.
All of this is made worse by the fact that matchmaking is typically stapled to games that aren't even really fun and mostly persist through the sunk cost fallacy, so people end up projecting their dissatisfaction with the game at the playerbase, after all the game can't be bad since they've sunk hundreds of hours and dollars into it.