Because you can’t read the article by reading it’s number, and there isn’t enough room for the whole article on normally visible parts of the body (not to mention the cost of that tattoo).
The QR code would give you a way to copy/paste the id so you can actually give and, you know, read the thing.
Spotify can actually do that, but it’s a bit cumbersome: as the local songs to a playlist on a PC. Then set that playlist to be downloaded on the phone, and it will download the files directly from the PC as long as they are on the same network.