> Prepaid SIMs are already manufactured, packaged, and shipped to convenience stores, so why not a Prepaid eSIM QR code?

Unlike regular SIM cards, the eSIM QR code doesn't actually contain your subscriber information. Everyone who uses the carrier gets the same code. This can make the process of signing up for a new carrier more complex, as you need to tell them your phone's IMEI number before you attempt to use the SIM. (This also makes switching devices harder; you can't just pop the SIM out and use it in a different phone.)

So a post-eSIM prepaid carrier would probably end up selling something like iTunes gift cards, with a code you have to scratch off and type into a website. Or they'd just sell exclusively online.