Markdown is useful for editing, but completely irrelevant to reading. So there simply is no demand for such things. It also greatly limits formatting options compared to the editors that authors and publishers already have, so they would be adding restrictions to their own work by pursuing Markdown.

In general, Markdown's use case is for easy editing of basic formatting in simple documents. Perfect for notes and small bits of documentation. Not perfect for many other things.

I'd say that the overall demand is too low. Plus, aside from self published books, I'm not sure how many publishing houses (of any size) use/support Markdown as a format for their authors.

PDF and Epub support a featureset that far exceeds Markdown in it's basic specification. Translating from one to the other would be pretty lossy, but possible.

You could probably use Pandoc to do this yourself, but again I'd stress that Markdown is really not meant for rendering PDF or Epub content: https://pandoc.org/

> Given the rise of Markdown for personal notes and technical documentation

This is your problem. You’re thinking a mole hill is a mountain. Tech dorks liking markdown doesn’t mean much.