Yeah that would be cool and in a free-er market more friendly to healthily competitive innovation diversity, we'd already have natural-sounding narration built

Yeah that would be cool and in a free-er market more friendly to healthily competitive innovation diversity, we'd already have natural-sounding narration built into every browser and reader; which would be an accessibility UX boon. But the publisher oligopoly wouldn't stand for it and there's not really much of an incentive for the marketplace monopsony-monopoly janus/jani to bake it into their products for free or a flat fee, even if the lawsuits from rights-holders standing to lose out on audiobook sales would be worth swatting away.