I'm wondering if you watched the video yet? It spends some time on exactly this point.

I misread the poster's question. Yes, this video does answer his/her question as stated. I thought s/he meant why signal shape and frequency variations at very high frequencies don't matter, which I answered. At 20khz you'd have an extremely hard time noticing any difference between a sine or square wave, or the different tone of 20.5khz vs 20.1 khz. Even though as the video shows, the wave retains full fidelity and detail at 20khz, he's zooming in to show that. Our ears can't "zoom", they hear it at the super high, tightly packed shape. The logical extension of this is why a square wave at 30khz sounds exactly the same as a sine wave: it's silence to us, thus the same.

Interesting point. I hadn't considered that the sounds of the waves would converge at higher frequencies.