I could see one reason being that it would make them harder to protect with digital copyright protection, but plenty of textbooks or books are already offered as PDF (protected or non-protected). A similar copyright protection could be added to a text file such as Markdown. Another reason could be that a 10+ mb Markdown file feels unwieldy, but it would be easy to lower the filesize by separating the book into a multiple .md files by chapter. For example, I recently bought a 300-page PDF Kaplan textbook for a financial exam I'm studying to take. I would have much rather purchased a set of Markdown files that I could easily add to note taking software. Is the overall demand for this format too low to justify the support?
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.
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/
This is your problem. You’re thinking a mole hill is a mountain. Tech dorks liking markdown doesn’t mean much.