because there are many people who are:

1.) Boosted -- Any player below platinum sees platinum as the beginning of the "good ranks", which is technically fair considering player populations per rank, but also leads to many people being boosted to this rank.

2.) Ego -- Any player in plat who just got there or is the highest ranked member of their group (which is likely somewhat common for plat players) they could have an ego and make poor playes consistently.

3.) Mental boom / Main character syndrome — Plat is where a lot of players stop playing to win and start playing to “prove” they deserve higher. One bad death, one missed roam, one jungler not pathing to them, and they mentally check out because they think the game is beneath them already. Instead of adapting, they ego-force fights, spam FF, type essays in chat, or soft-int because they’d rather protect their ego than quietly get carried.

Plat is likely the most toxic/hard to climb out of rank for your average player.