What's going on in diamond+ that slows down skill growth in NA?

Not a troll topic, serious question

NA Solo play, bronze-challenger is much lesser quality than the korean server and china server, and some would even debate even lesser than the EU west servers.

So here is my question, To climb in a elo, it's a very simple process that is easier said than done. You observe yourself, you listen to advice, you acknowledge what you are doing wrong, and you correct it. Let's say your key problem is, you legit never ward, you correct that problem, and a few other things and you begin to win more games. Let's say your problem is, you are just STUPIDLY over aggressive, tower diving level four and failing, so you stop doing it, and you find the correct form of aggression and you start winning more games.

That's once again how climbing works, you are low elo, you realize what you do wrong, so you make a effort to fix those mistakes, in all hope you have made it to diamond/masters/challenger, for you corrected all your mistakes.

However here is the thing, we watch people in diamond, masters, and challenger, specifically NA players, and we see them make such low grade mistakes that people at that level of ranked play SHOULD not be making. We see them forgetting to ward to avoid ganks, we see them playing overly aggressive, we see them doing just simply silly mistakes that high elo korean players just don't make.

So what is the cause of this?

Do people in Diamond just enter this state of mind where they think "i basically beat the game cause im diamond, i have no reason to try hard anymore?" for when it comes to all my diamond friends, it seems the only times they actually try is when they worry about demoting.