If you are thinking about why the game engines aren't broken down into finer reusable components (e.g. one module for rendering and another for physics, etc.), it probably has to do with how these things are tightly integrated, and how every game engine has a different design philosophy on how the world should be organized. The different organizations might also mean that you have to design the games to fit a specific game engine, which is unfortunate.
Maybe you are lamenting why the marketplaces or asset stores are so closely tied to the game engines? For some plugins, they have to be tailored to the specific engines. But for arts and sounds, there are many free and paid options to choose from, although not necessarily of the same quality and cost as one might get from the asset stores that are sponsored by the game engines (e.g. Unreal Megascans).
There are still various tools associated with game development (e.g. Blender for 3D, your favorite text editor for scripting), and those aren't tied to specific game engines.
If you are thinking about why the game engines aren't broken down into finer reusable components (e.g. one module for rendering and another for physics, etc.), it probably has to do with how these things are tightly integrated, and how every game engine has a different design philosophy on how the world should be organized. The different organizations might also mean that you have to design the games to fit a specific game engine, which is unfortunate.
Maybe you are lamenting why the marketplaces or asset stores are so closely tied to the game engines? For some plugins, they have to be tailored to the specific engines. But for arts and sounds, there are many free and paid options to choose from, although not necessarily of the same quality and cost as one might get from the asset stores that are sponsored by the game engines (e.g. Unreal Megascans).
There are still various tools associated with game development (e.g. Blender for 3D, your favorite text editor for scripting), and those aren't tied to specific game engines.