I don't understand why. Tetris is an all-time classic game, which probably requires a lot more skill than League. Don't get me wrong, I'm not saying that League doesn't require skill, I'm just saying that the top Tetris players are mad insane, their decisionmaking is very fast, like perfect accuracy, reaction time, and they don't feel human to me. I can't understand how they can process all the information in such short time, and also keep up the concentration for over 10 minutes without a single break. In League, you have a lot of time when you're not focusing that hard, even if you're a rank 1 player. Like your mind can take a little break when you recall or die, but in Tetris it's just nonstop pure concentration.
Tetris is also a competitive game, there are many tournaments, and in Tetris, if you screw something up, you can only blame yourself, and the game entirely depends on your skill, while in League even Challenger players can lose in an Iron game if their team sucks that much.
Tetris is also way less forgiving if you make a mistake.
So why, just why do people use it as an insult?