> but they are so pervasive that everyone knows how to use them.

Nope. I know people first-hand who don't. Hell, even I don't know how the average person does it. My own phone's camera app doesn't detect them so I had to find a random 3rd party app and trust that it's not stealing the info. In fact I'm not sure I've ever used the stock camera app on any phone to scan a QR code before. For some folks it might be "press Camera and scan", but for others it's way harder than it looks.