>Why can't I write a desktop application that just asks the OS for the users preferred browser and provides it with HTML/CSS and UI interaction callbacks / events? Render it in a native looking window.

You can:

- webview (I mentioned it elsewhere) https://github.com/zserge/webview - sciter https://www.google.com/search?client=firefox-b-1-d&q=sciter - ultralight https://ultralig.ht/

Granted it's not "user's preferred browser" but it fits the bill. They all have issues, largely solved by Electron packaging the browser engine with the binary, and even that isn't perfect.