How is it harder to parse than a PDF or a docx?
Imagine it wasn't decided on Markdown. E.g.: I don't use markdown when making plain-text notes. I make it all hierarchical and indent using "- ". I.e. headings are top-level, sub-headings are indented by one, and so on. Eventually, you realize that all things can be represented as just lists of nested lists. No need for some arbitrary "heading" type, just nest it all based on the concept.
Obviously it's not harder to parse than a PDF.
If someone told me they needed me to parse a "plain text" file, to me it sounds like they'd want me to provide some statistics on an unstructured text blob.
Free form text does have properties, such as byte length, character count, word count, line count, sentence count, paragraph count, and so on. Most of which is simply variations on whitespace delimiters.
But an individual plain text file... all on its own? Could be a shell script? Could be a newspaper clipping? Could be a dictionary file? Could be an array of XYZ vertices, edges and faces? Could be all of the above, and an email signature at the end?
Someone tells me: "here have this plain text object," and I presume it is a monolithic blob of ASCII, if I'm lucky, and maybe it's War and Peace or Moby Dick expressed in emoji if I'm not.
Well, most would agree markdown is a plain text format for example, but it's not totally unstructured.
Still it's a bitch to parse compared to something like JSON.