As a Plat level player myself. I think the reasoning behind this is that, we're better than the general population if you're a true blue plat player(not boosted or anything). However you lack the true difficult mechanics to reach Diamond. Thus the only real way we can attain Diamond is via; Winning streaks with lucky teams, or giving up our social lives and playing League until we grasp the difficult mechanics.
Plat players are usually players that KNOW what to do, so they can tell you what to do, but put in the same situation we couldn't execute the move perfectly either(even though in our heads we would like to believe we can). Players in Gold and Silver, lack a lot of simple things like when to use your cooldowns, positioning, basic mechanics, etc etc....however Plat Player tend to have the basic mechanics down, and it's hard to grasp the idea of anyone in Plat division to not know these things at this level(when to split push, when to group, when to ult, and not to clump against a malphite Ori Yasuo amumu team).
So an example would be someone is running away with a small amount of health and you're just like "JINX ULT ACROSS THE MAP AND HIT HIM" you expect the player to be a Plat Tier, or high level player to make that shot, but we probably wouldn't make it ourselves.
In addition we fail to understand that just how we expect to be carried to Diamond without getting the proper mechanics, many people are boosted to Plat(because it's the one tier when you actually become good) and a lot of people are carried to Plat. When we run into these players who quite obviously don't belong here. It can be incredibly frustrating. Especially when they bring the low elo toxicity with them(difference of high elo toxicity and low elo toxicity is high elo toxicity is just yelling but they still try and want to win the game, low elo toxicity sabotages their own game, you cannot mute them they will ruin the game)
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TL DR; Plat players have the knowledge of the game but lack the hard mechanics to be Diamond, they know majority of their games will rely on their team so they focus on what their teammates SHOULD know to do by now. If their teammates don't, they simply do not have the mechanical advantage to single handedly carry games like most True Blue Plat players could in Gold and Below. Just being as real as possible with you since you're looking for an answer**
