> Looking at one's feet is also inefficient since you don't dance with your feet.

What do you dance with, then, and why there are step diagrams (with lines and little shoe prints) for classical dances?

You dance with your entire body. You do to an extent use shift weight to parts of your foot and use toes a small bit , but the ankles, knees, thighs and waist are all more important than the feet if we're simply focusing on lower body motion.

Re: the gotcha question, because these are simplified diagrams for teaching beginners or communicating genre-informed choreographies in shorthand. Even then, nobody achieve's any dancing simply by moving their feet as the diagram depicts (and they couldn't possible do so without the rest of their body). Part of the reason for their success is that people are already familiar with dances and combine the diagram's directions with the understanding that they must achieve it with a full body vocabulary.