[andybak]

1. Unity projects mainy use C#. So that automatically limits the frameworks you can switch to unless you want to port to a new language.

2. A fair chunk of your code will be calling in to the Unity runtime to use functionality provided for you. This may not exist in other engines or exist in very different form

3. Unity controls the gameloop and the renderer. Other engines might have very different architectures and constraints.

4. Abstractions cost performance. You're usually trying to hit a very tight millisecond budget per frame so adding extra layers of abstraction is not good practice.