You need to take into account that nobody can perfectly read a book first time. I've done a reasonable amount of short-fiction narration for audio magazines, and even with many hours worth of script reading under my belt I still fumble every third or forth sentence. So I need to re-read those, and then I (or somebody else) needs to find those errors and edit them out, replacing with the fixed audio. Then people need to listen to the entire file at least once to ensure the whole thing makes sense and I didn't leave out a sentence somewhere or something.

Audiobook narration is one of those things that is remarkably labour intensive, certainly much more than I'd have guessed before getting into it.