Champs: Why do they even exist?

I'm not talking about some philosophical debate about why they exist, I'm talking about why certain ones are allowed to exist in the game with the kits they have. These Champions are impossible to balance for healthy gameplay, and yet they're buffed to infinity or given new low-risk high-reward abilities that literally change League to focus around them. A few examples: Skarner, Karthus, and Veigar. Not all of them, just examples of a few. Skarner's Passive is impossible to work around because no one that doesn't own Skarner knows what the hell the crystals are for. Adding in an ability that stuns-and-drags an opponent Champion (leaving the opponent inactive, yet able to still be damaged just as badly as normal gameplay), and Skarner here has a winrate higher than Zed's (reaching very close to 60%). Karthus, while having a slightly less average winrate, has abilities that are just unneeded for healthy gameplay. While his Q and W are balanced, his E and especially his Ult are just toxic to play against. A very wide AoE burn and a Global that can take up more than half of all opponent's Health? I mean really Riot...I don't care that it has to charge for 3 seconds and can be interrupted, if you can't reach him in time he'll end up getting an effortless and faster Pentakill than a Katarina surrounded by squishy feeders. And if he doesn't, his team nearly always finishes up the job and quickly does an early Ace. Lastly Veigar. I don't really mind the CC, but when you add that to a Passive that allows his AP to infinitely increase, it just rubs me the wrong way. Mid? Great. Top? Even better. Support? It's like he was made for it. Even if he decides to go Support, he'll always be the Carry of any game because his AP just increases to the point where he kills everything effortlessly. It's like one of Riot's employees was playing with his tea cups and thought "You know what'd be cool? An evil wizard who doesn't give a fuck about anything. We should make this a Champ!" And so they did, and he continues to demolish any late game would-be hopefuls simply because Fuck You. I know this seems more like a rant, because it is, but it's like...do they even think about what they're doing there? First off the betrayal we're facing right now with Solo Queue never coming back is an atrocity, but it's not that bad compared to the fact you can't have a successful game in Ranked without 1) going in a full team of pissants who constantly send you game invites when you log in, because it's either that or you vs 5-man premades, and 2) having to pretty much master two roles before you can go into Ranked or your team will report you and you'll pretty much lose every game. Oh, by the way: starting Patch 6.11, you're no longer guaranteed either of your two roles. If your Queue timer "takes too long", you'll be forced into an autofill role which places you into any role other than the two you voted for. Want to Top or Mid? Nah, it's 5 minutes down the line, it's taking too long so we're putting you in Support instead. Better not feed or you'll lose LP and get reported~