Considering I can now press '.' on GitHub and get vscode, not including codespaces or opting to run in vscode on the desktop, why would I use one of these alternatives? And to be clear, I'm not advocating anything, genuinely curious what makes these better then the alternatives.
You are thinking of personal use cases, but the point of these editors is that they strive to be as light weight as possible and are meant to be part of some online service that for some aspect of their functionality/features includes offering an embedded code editor. VScode has too much stuff for most of those use cases where you'd use something like CodeMirror.
vscode is built on top of Monaco. These are all lower level editors with very well exposed APIs that let you build on top of them. It's more for developing something with a code editor versus directly using them everyday as your editor, though I'm sure people do.