I've been flamed countless times(I main jungle) for no reason and it's fine, I get it, people get frustrated but all the time, almost every game?
For everything, even the little things like missing cs, getting ganked or losing lane to a better player, and then they vent on teammates and that causes a chain reaction, worsening everyone's experience.
Maybe someone had a worse day than you think and raging only makes it more so. People just don't think about that and don't care. People can be very selfish. I've been there beacuse a lot of times I play to get my mind off things irl, and many probably/possibly do. Riot even implemented the "parental control" system for underage kids who are exposed to this kind of thing.
Of course this doesn't apply 100% of the games, I've also made a similar thread about this, but I still don't get the what's the root of this problem.