Because after a while seeing "lol this soraka is so trash" after 1 or 2 deaths starts getting to me
ontop of that it dosetn end there even if you mute said person(s) they will keep harassing you or trying to till the game is over and it often ends up in a loss because said person(s) are to busy trying to flame you and get everyone else to do it and most of hte time everyone else will do it
the system doesn't separate feeders into their own quarantine so most people just assume the worst, don't blame frustrated players blame the matchmaker (riot). No one knows if you will be literally standing in front of the tower tanking damage cause ??. You could be a good one, but who knows? The doubt and suspicion is created by the game itself. If players that feed were isolated into their own pool based on stat-averages then the frequency would be lower, people would not suspect you so early, and toxicity would reduce.
Realistically you should not be dying as a Soraka 2v2 unless you mess up. And after so many games of playing the champion mistakes should be minimized.
