because we have plot armor
Plot armor is selective and inconsistent specifically to forward the narrative (i.e. they'll be weak when the author wants tension and strong when the author wants triumph). If they're just consistently too powerful for anything in the setting to threaten them, that's not plot armor - they're just powerful characters.
Some inconsistencies can actually be consistent, however. As an example from a series that gets strong characters right, in One Punch Man, Saitama - a man who nobody can even threaten - is unable to kill a mosquito. This might seem inconsistent at first, but in reality it's an expression of something more fundamental to the character that overrides his insurmountable strength: nothing in the entire universe can threaten him, but he's completely at the mercy of the mundane inconveniences of daily life... like mosquitos. Or getting the date wrong and missing a sale. Or being so short on cash he develops a habit of hunting for sales in the first place. Or going bald. Etc.