It's because YOU CREATE THEM YOURSELF. I can't tell you how many games i'm playing in where one player is doing poorly, then someone decides to just rip into them for NO REASON AT ALL, causing the player they're flaming to just give up and stop trying. Honestly, I rarely see people who just queue up to troll. That's like a 1 in 1000000 thing. However, I do frequently see people who stop caring after their teammates continue to flame them. And as someone who queues exclusively for support this is the worst feeling on earth because i'm not playing champions who can 1v9 if say my ADC just decides they've had enough.
This is the tangible effect flaming has. This is why riot bans for flaming. It doesn't matter if I mute them, someone on my team is trolling and there's absolutely nothing I can do about it. They won't listen to me trying to calm them down, and all of this is because someone decided that they were going to be a terrible human being for absolutely no reason. I'm not condoning people who give up/soft int, i'm just saying that it would not be as big of a problem as it is if people stopped being terrible towards anyone having a poor performance.