It is true that your ears do not perceive the higher frequencies in the same way (through pitch detection). However, if you put on a headset and apply only frequencies above 16kHz, you will distinctly notice a change in the pressure in your headset's ear cups.

Good point about the savings. I was using uncompressed format as the reference, but it is indeed unlikely that YouTube serves out lossless audio.

I also should have used the word "delivery" instead of data storage. Those are two separate problems: where the original asset is stored (and how, if they don't store raw originals), and also how the asset is delivered over the web.