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
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.
If you're looking at the code locally, what editor do you use that doesn't have a markdown viewer? I use Sublime with MarkdownLivePreview. Although honestly I generally don't, because md is so easy to read without a viewer.