Markdown is a text markup language, of course. The output does not look like the input (unless you want to read raw text Markdown, in which case LibreOffice works fine).
How would that look in a single-pane, edit-in-place, wysiwyg editor? Where would you type the input, and where and when would it show the output?
These things generally already offer side-by-side page layouts anyway, no?
Doesn't seem like that much of a stretch from a UI perspective to do something similar with a Markdown preview.
Honestly I always prefer the "non-rendered" version, it looks way nicer and readable to the eye.
Obsidian does this. It displays the raw Markdown for the line under the cursor, and renders the marked down content everywhere else in the document.