Publishers can greatly increase potential profits on their content by making a modest investment in audio production, and thereby controlling the distribution of that audio, as opposed to exclusively licensing it and accepting fractional revenue.
Consider the fact that the same distribution channels that are selling eBooks are also selling audio. For a publisher, controlling your own audio and selling it through these channels can be the difference between dollars and dimes.
The table below spells out a hypothetical risk vs. reward scenario on a typical project: