You have to balance around players who actually know how to play the game and use champions appropriately.
Otherwise, you will get abuse cases ruining higher tier play. For example, if Azir was balanced in the hands of a Silver player, he would be absolutely bloody broken in Diamond+ when somebody competent used him.
If a champion is strong in lower ranks (i.e. Bronze, Silver, Gold) but weaker in higher tiers of play, that generally means that sufficient counterplay exists to the champion but that lower tier players just don't know what it is or how to implement it.
Honestly, I'd be happier if they focused on Diamond+ for better competitive balancing.