Markdown is a plaintext format, that's what makes it great and got it adoption, it works without formatting! Rendering markdown is a nice to have, a luxury that gets very complex very quickly when done properly. If you want it, install a an application that does markdown rendering, I use Obsidian.

The fact that a file can't be opened as plaintext in a plaintext editor is not a problem with Markdown, it's an OS problem that effects any file you'd want to open as plaintext.