IMO a huge reason for the current lack of balance is partly due to the playerbase

Specifically how both the players and Riot handle PBE. PBE is supposed to be the public test realm, where players can try out the changes that the devs make, in theory to check for bugs, test for balance and provide feedback.

But the vast majority of players on PBE only play there because they can get free skins and champs while not having to pay or grind for them. In almost all draft games on PBE new champs, VGUs or heavily rebalanced champs are banned during the draft phase. Players dont play the game to look for bugs or balance, most play because it is league without the grind. Nothing is expected of players using the PBE.

The blame also falls on Riots balance team and their policy towards the PBE. For almost its entire existence Riot has ignored the PBE and players responses to it. Once in a while a bug will be caught before a patch is shipped, but in a vast majority of the time the minority of players who use the PBE to find bugs or test balance will find a bug, report it and the bug will still make it to live, possibly taking months before it is finally patched out. Almost 0 balance changes are made from the communities PBE findings, Riot instead going to reddit where the larger reddit community will read of the PBE patch notes from a third party and make their complaints based on that.

Which leads to things like Zoe, 7.21 AP Galio, Klepto, IE+shiv vayne with Q combo, etc.

Sure the balance team has make some "questionable" balance changes, but an even larger reason for the current lack of balance is that there is no real beta balance testing. Everything is done in house in alpha, which is how some things make it to live that are completely busted because, even though the live balance team believes that they are balanced, they dont have a Vandiril stacking AP on galio to one shot a tank, or testing the new changes on Morde.

TLDR - Their needs to be some sort of beta balance feedback taken into Riots game balancing.