Why are the communities of competitive games so notoriously toxic? Culture. I think books could be written about the 'cultures' of certain games, but basically, the culture of competitive games places too much value on rank and prowess. And this has a few consequences.
People react to this cultural pressure in different ways. The way we see this most often is people throwing temper tantrums at teammates for poor performances - the logical end-result if someone's worth is largely based off of how well they play. Or maybe they feel cheated of a rank, or number, because the matchmaker put them with someone who they consider lesser. This is a direct insult to them - seeing the game subtract from what literally defines their worth because of someone else feels awful.
This is where what I call de-'sweating' comes in. It doesn't seem to me that there is competition for the thrill of it anymore, but merely to dunk on the other dude across the screen. I don't know realistically how this happens, but the emphasis needs to move, or we get situations where kids sling racial slurs at each other because they couldn't win, and taking that therefore as an insult to their self-worth. Or burning themselves out at age 19 playing 9 hours a day trying to improve to a certain level, so they feel better about themselves.
Ladder anxiety is another common manifestation of this. But that's not the main concern, which is that the laser focus within these communities on your ability to play the game well, and not on competition itself or sportsmanship, fosters the awful environment within these games. De-'sweating' just means rank doesn't matter. It should be a tool for fostering fair competition and even play.
lunchtime text dump over
Here's my two cents.
Think a lot of people don't have much control over their lives apart from video games. When your teammate fucks up when you're having a good game, it removes that element of control. Also people don't have proper goals and ambitions out side of the games, so they become hyperfocused on ranking up and get weird and obsessive.
Also, free time. This is especially true for me personally. If I get home after a long day and just wanna chill to some vidya, but someone else griefs it, they're wasting my limited amount of time. Especially in competitive games where you can't just quit and go find another match, or where you know your team is doomed from the start but you still have to play out a futile 40 minute game. That shit sucks. I've never been a rager, and I can't be asked to flame my teammates, but whenever I've said 'fuck this' and shut my computer off mid game it's because of that.