> You find MD files in every open-source project and lots of other places, but almost no viewers

Huh? Any open-source project on GitHub, at least, has the viewer right there. It's the default view of markdown files. I assume other repos are similar.

E.g. The readme https://github.com/jquery/jquery

Do you not realize you're in a browser, on a Web page, if you're on Github?

And do you never actually clone the project to your machine and use it? When I sit down to actually use the project and I want to read the documentation, I want to double-click on the MD files and READ them. Not edit and preview, but read... with their intended formatting.