It is because the primary target for markdown is what would in any other formatting language be the source. Markdown has no source. It is guidelines for good looking plain text layout that when followed can be used to make a typeset document.
Everybody sort of knows it sucks as a formatting language. But we love it anyway. The benefit of being able to get a typeset document from a nice looking plaintext document is just too convenient. It is why specialized markdown editors make no sense to me. Why would you target something as shitty as markdown if you have a specialized editor? But really, if you at all care about the semantics of your document don't write it in markdown. But more importantly please don't kill the utility of markdown by adding semantics to it, all that noise just make the plain text look bad.
> Why would you target something as shitty as markdown if you have a specialized editor?
Your at-rest format allows you to use any tool to edit it. The specialized editor becomes a preferred tool, but not the only tool nor the most important one. Markdown and it's ecosystem came before the editor.
> Why would you target something as shitty as markdown if you have a specialized editor?
Because Markdown is the standard text formatting language. It’s the format everybody works in.
> But really, if you at all care about the semantics of your document don't write it in markdown
But it’s the standard text formatting language everybody uses.
Unfortunately, the plain text does not look good, and does not offer good control of the typesetting. At least, nearly all markdown I see is almost unreadable in its “raw” state. I’d much rather read manually formatted, monospaced, text. That way, you can use asterisks as bullets, as footnote markers and as emphasis markers all at the same time, and anyone who is familiar with normal typographic conventions will understand what is what.