I think part of the reason why people want to pretend Zed is OP is because...

...it subconsciously relieves themselves of the mental ordeal of having to accept and understand that he can defeat them if he plays more skillfully than they do.

So they push him behind an ethereal veil of "OP" within their minds, so that they don't have to accept things. Or accept themselves.

It prevents them from having to take responsibility for their play around him. Or their skill level.

And since Zed looks the flashiest, people don't act this way towards any other champion. They don't want to feel like they've become part of a montage. They don't want to feel like they need to improve. It's easier for them to shut improving out of their mind when they think they're up against something that they cannot do anything about, when ironically that something is an assassin with the absolute highest amount of counterplay in the game.