I see a lot of messages framed a lot like this, so I want to tackle them all right now. Why hasn't Riot nerfed Amumu yet, or why don't they buff Syndra a little bit? Simple questions like that are framed in such a way that you're trying to get people to agree with you, and that's bad. Here are four reasons why Riot hasn't done that yet.
1: Not Enough Time
Riot may be a huge company now, but they are constantly having to work on making skins and champions to stay in business, host tournaments, and do a lot of maintenance and work behind the scenes. They have to make the best use of their time, so they can't hit everything at once. While it is greatly needed, a Karma rework may not be coming for a while because not a lot of people play her and because they are so busy making sure the game is still balanced.
2: Afraid of Over-doing it
Remember Rengar when he was first released? I would laugh whenever I saw an enemy Rengar, as even after people happened upon AP Rengar, his win rate was still an abysmal 40%. Then he got buffed, drastically, and became the bane of every single person's existence, so they nerfed him not once, but twice to keep him in context. He's actually an example of what happens when you go too far both ways. He's back down to his lowly 40% win rate, or something close to it. First they over-buffed him, not expecting the power to come out from his passive and ultimate, and then they went too far in nerfs.
Urgot back in the summer was also a force to be feared in professional play. He was one of the best champions to counter the typical ad carry setup in the bottom lane. As a result, he was nerfed, a little too hard. Now, he's not doing so well, and he will have to be addressed.
3: The players are changing the game every day
Remember the days before Pulsefire Ezreal was announced and became a big deal? Back then, Ezreal was a situational pick, played by very few and mocked as being a bad ad carry because he didn't fit in to what we typically see as an ad carry. Then, people started to play him again with Pulsefire Ezreal. People found out that his attack speed debuff was the best thing ever, that he could fit into a wide variety of team comps, had great mobility, and was an awesome champion. Even after the attack speed debuff has been taken away, he still sits up there as the most popular champion right now. Something as simple as a skin release can spark a huge change in the player base, and a simple bit of publicity, or pretty much anything, can change how we play. The meta may stay fairly consistent, but what goes on in the meta can change drastically, and no game company can keep up with all of that.
4: It might not be their fault/might not be a problem
Is Darius really overpowered? Or are you just letting him get away with ulting your entire team because you can't keep it together? Darius is a really controversial champion in terms of the OP craze, but is he really overpowered, or is he just exploiting the lack of unity in most solo queue teams to murder them all. Even with all that murder, is it really helping by taking away gold from the team's ad carry? Think about whether a character is really overpowered, do your homework on LoLKing and in-game, and then post if you think there is a problem.