1. You can have both. Parts of our site (Our World in Data) the researchers edit via a GUI, but under the hood it is making a commit and pushing to GitHub. My guess is MDN just hasn't gotten around to that yet?

2. GitHub and GitLab are a lot better nowadays at allowing you to edit markdown docs via the web and preview the results in a few clicks. Not quite as frictionless as 1 click, however if you are an editor and are constantly bombarded by spam, probably saves a lot of time as you know the person who is making the change and they are forced to add an explanation via commit message.