Streaming the raw data would also be more flexible for other use cases. From what I've read, it was streaming off the generated recovery passphrase, but users aren't required to generate one--they can input one. That'd require a much more sophisticated algorithm than simply reading from the screen, since users are using two buttons to pick from hundreds of words in a randomized menu.

And besides, Ledger firmware can be updated to change fonts and such so it'd be long-term safer to just stream the raw data.