Just opening in notepad by default works fine. The point of markdown is that it doesn't need rendering to be readable. If you need more, you already have a preferred Markdown IDE setup anyway.
Markdown is pretty terrible as a rendered format anyway. The standard for Markdown itself is extremely loose so we invented several types of Markdown, indistinguishable from each other with slightly different features and syntax, to cover the loose standard. CommonMark isn't Markdown yet I see nobody saving files as .cm to actually indicate what kind of Markdown they need.
This is a major annoyance in messaging apps, slop tooling, and of course git GUIs. Building a standard viewer will bring all of those problems to the operating system, and for what? To annoy the people who already have a Markdown viewer of choice?