Wouldn't it be great though if they did? Imagine if all note taking apps were just a fanciful way of creating standalone HTML or SVG files. Note-taking could then greatly benefit from existing technology.
Does something like this exist? And if not, why not?
So I would say that you are right - vendor lock in.
While not open source, there's also Styluslab's Write which saves the files as svg so you can open them anywhere.
It did make a switch from iPad to Samsung Tab a breeze.
“The TextBundle file format aims to provide a more seamless user experience when exchanging plain text files, like Markdown or Fountain, between sandboxed applications.”
A bunch of apps support it. There’s a list of the front page of the website.