Not every player who builds casually wears the same hat in game, and just because they build at a lower power level doesnt mean they dont want to play optimally in game and enjoy that interplay
Having a big expensive threat can be a liability in Modern because of cards like Path to Exile and Terminate, but sometimes they dont have it
They don't copy whether that permanent was tapped or untapped, whether it had any counters on it or Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on
It's possible for Mana Breach to come down as early as turn one, making it practically impossible to play the game for multiple turn cycles
Faerie Mastermind can do more than its fair share as it leeches off of multiple opponents, too