Flac files have most of the benefits of streaming, whereas physical books are a lot less like digital books. You can instantly purchase and download flac files, sync them across devices, play them over networks, duplicate them, hold thousands of them on your phone, etc.
Also I can shrink my FLACs into 256kbps OPUS and store ~9 hours of audio in 1GB. I cannot (easily) shrink my physical books into microfilm and read them.
The better comparison is PDF + OCR’d text VS Kindle.
