When you can program inflection into narrator software. Tell it where to slow down, when to emphasize a point, and wait for the next line to exactly match the next scene transition. For audiobooks it will be a lot more involved because they're more like actors. You have to program different voices, broader range and more nuance in emotions, and probably the ability to interpret more creative formatting in the book layout.

On the other hand, https://www.theverge.com/2019/8/23/20830057/amazon-audible-s...