Now if two duffers with zero experience can manage it (in the middle of the night) why can't an experienced crew with three or four captains on board!
Because you only tried it once. Probabilities.
If you and your duffer mate had run, say, 5000 sim sessions can you confidently say never would that mistake have gone undetected?
Or, if 5000 similar to you people had been put on a sim would none have committed this blunder.
One point, often unappreciated, is that when you put in as many repeat events as in commercial aviation, even relative low probability "stupid" errors are almost a certainty. Given enough time.