It's not really possible because a game engine works differently from a library.
A library is a component that you add to your application. While it might be opinionated in the way it presents its interface, you can typically build some kind of abstraction layer on top of it and swap it out with something else in the future.
A game engine basically is the application, and your game builds on top of it, filling in the game logic, assets, level design, etc. The earliest game engines like Unreal Engine 1 were basically just the game Unreal with all the game-specific bits stripped out.
An engine is more than just opinionated. It determines the general application flow and structure, how each component is conceptualized in the architecture and how things connect. It even determines which programming language you can use. You also just use a lot of components from the engine: rendering, input handling, physics, animation, networking, parallelism, asset processing, etc. Things of that scale would probably be separate libraries for most other types of applications.
Beyond programming, much of your work will be in engine-specific formats that simply cannot be automatically converted to another engine: project files, level design, component connections and settings, graphical programming and shaders, animation state machines. You could design all of this in your own custom formats and build it programmatically, but why would you take that development overhead when engine's editor already does it so well?
That's not to say that porting from one engine to another is impossible. Assets like graphics and audio can be moved with no or minimal adjustment. Game design is still the same, and typically most concepts are similar enough between engines that you can do a line-by-line conversion for most of your code. And some games really do use Unity more like a rendering and input library, Caves of Qud is one example. But those are rare and most games will simply require a lot of elbow grease to shift engines.