There are too many unique champions
That's what causes certain champions to be broken. It's not stats or mechanics, it's that those mechanics cannot be countered or matched with skill alone.
In competitive play, it's always baffled me that the only true test of skill is not allowed. And they force imbalance by removing the option of a mirror lane.
But this isn't about mirrors, though allowing them would solve this problem.
It's that there aren't enough champions with certain abusive traits that can match up to others.
No one else has Yasuo's 10+ dashes in a row and complete projectile negation. No one else has Vlad's infinite costless sustain, No one else has as high-damage an AoE stun as Kennen, and certainly no top laner has it. No one can replicate Twisted Fate's ultimate, or Shen's.
If there WERE champions who could match those strengths, then they wouldn't need to be banned. We wouldn't need bans at all if Riot would create champions that can match up to ones they give extremely specific strengths.
You don't see specific Engage-tanks getting banned unless they get a stat boost that makes them better than the other choices. Any Amumu player can just as easily pick Malphite or Sejuani. That's one example of a class that has plenty of options for the same basic role, that can be provided regardless of the bans.
