Why don't Bookshare books contain recorded human speech?

Some DAISY books have human narration, and many do not. Bookshare books only contain digital text.

DAISY format is an extremely flexible file format that permits both digital audio (such as recorded human speech) and digital text (that can be read with any of the available text-to-speech engines and synthesized voices).

Digital text can be synchronized visually with audio, for simultaneous reading and hearing a word read out loud using one of the many very high quality voices. The text in Bookshare DAISY files can also be enlarged for people with low vision, as well as being supported by most Braille displays. These differences give readers with print disabilities choices over how they want to read books.

Bookshare has chosen to produce DAISY digital text for the benefits of rapid production of accessible books, so that Members can obtain accessible books as quickly as possible. An additional consideration is that DAISY digital recorded audio files require up to several hundred megabytes of disk space which would make downloading slower.