> Why is this also suboptimal, or why does this not work as well? Why is the approach by qutebrowser better?

Building a browser extension will always have limitations.

For example, if you download a file, you usually don't have a way to open/delete/retry it through a keyboard command or an extension. Or moving tabs around between windows, change the order in the current window. You have to go back to using the mouse for many tasks, even with extensions.

Browsers also have a lot of minimal chrome UI which you can't get rid of with an extension. (a while ago, I did use Safari in a mode where it only had the very thin top bar, but it wasn't super practical sometimes because it was not the intended use-case)

Having qutebrowser built with the vim-like mode as first-class citizen gives more control over what can be done and how it looks.

I've been using it for a few years now and love the keyboard-shortcut centric aspect (I also use Vim) and the minimalism and customizability of the UI.