1. You cannot lose the game quickly! Other genres like RTS, games can last between 3 minutes or as long as 30 minutes. Not so with MOBAs, it's rare to see a game conclude before 20 minutes.
2. You cannot surrender early. Even if a game is clearly one-sided, you're forced to play it out.
3. Small mistakes mean you fall behind and feel weak/useless. It's a rat race/red queen scenario.
4. Your teammates get upset when you die to your opponents. Your opponent becomes a lot stronger.
5. There's no reliable way to come back from being behind. You're basically waiting for your opponents to make multiple mistakes, multiple times until the game is even again.
I think all of this adds up to a feeling of frustration and helplessness. Everbody knows this about League. It feels good to get fed because then the game is easy. It sucks to fall behind since it's such an uphill battle.
These feelings come out in team-chat. People project, they blame, they rage. And you can't really escape it, since you're supposed to just play the game out.