Well i mean...that goes back to League's core design, it was based off of DOTA, dota is a game where counter's are very important, not just very but almost to the point of extremes. In DOTA if you get counterpicked it doesn't mean you lose but if it counter's you it does it hard. Unlike in DOTA though in league you have Masteries, Runes and Summoner Spells, these helps to lessen your champions weakness or heighten their strengths. In league this works because each champion, well most there are definitely exceptions, is designed with strict weaknesses in mind that other player's can exploit with counterpicks but counterpicks are kept in balance by allowing the opposing player to select things that help them alleviate those. Builds are also a part of this, even if they do need to work on all these aspects League is much more balanced that other games of it's kind and many other competitive games as a whole.
For example, i'v been playing a lot of syndra lately, hyper mobile assassins usually counter an immobile mage like syndra, but if i take Exhaust and build correctly i have a higher chance of winning, not by very much but it puts it back over 50% rather than being 46-49%. Another thing is that in terms of leagues balance, people complain constantly about how unbalanced it is but the usual difference in winrates is within a 10% threshold, this means that outlier's that have say a 58% winrate are exceptions and those are target's for nerfs, champions that have 45% winrate are also targeted for buffs more often except in cases where the champion is getting an update or riot admits to their mistake when designing said champion so they want to keep them subpar(Yorick, Aatrox, ect). This also means that if we averaged up the overall winrates of every champion, that it would be around 50%, meaning the game is actually mostly balanced with about 15-20 or so severe outlier's at the top and at the bottom. Also, if we were to average up the overall winrates of every class, and compare it to the overall winrate for all champions in total, if it was bellow the overall average then that would mean the class is weak, if it were higher than average that would mean the class was too strong.
Now yes, some champions have very high play rates and that does affect their winrates because a champion shouldn't have a 30% playright and above 50% winrate, most champions that do have high play rates like 10% are either just under or just over 50%, such as Ezreal at a 21% play rate but a 45% winrate. On the other hand Ahri has a 53% winrate and a 21.6% playrate, which is waaay out of line. Lucian's is also absurd at 39% playrate and a 50.5% winrate, it might not sound like much but that means that 40% of player's platinum and above are winning games more than 50% of the time with lucian.
The first competitve game i played was Yugioh, Yugioh is not balanced at all, sometimes a certain deck will have a win chance of 90% against anything that doesn't have the same absurd winchance, that's called Teir 1. If a champion ever became that strong, then yes this game is unbalanced but at the moment, hovering around 49-51% overall winrate is pretty fucking impressive and means that with most champions in the game you have a very good chance of winning your game and can buff it up by your own player skill.