Champions will be released "overpowered" because people associated fun with winning. If your champion releases at 35% winrate, even if that is long-term balanced when looking at a mastery curve, that champion's playrate will never recover.

Another thing I see, "WHAT ARE THE PLAYTESTERS DOING? THEY SUCK AT THEIR JOB."

Okay, first of all, the playtesters are phenomenal an all high diamond to challenger (and there are even master-challenger designers on the balance team)

Second, playtesters have a few different jobs during the lifespan of a champion.

1. Assess the pattern of the champ (abilities, playstyle, role, etc) and make sure it is fun and doesn't have long term problems (Note they advise here, they don't make final decisions)

2. Help make sure iterations in development align with the designer's goal. Easy.

3. Help roughly tune the champion for release. (note this isn't perfect balance, but rather projecting the winrate for players with 30, 50, 100 games and how the winrate will be on day 1 and so on

I probably missed something but those are the big points.

TLDR champ design, balance, playtesting, all hard jobs. I guarantee 99.9% of you all would be awwful at them. Give them a chance, they all want to make the game better because they love this game as much or more than you