This is because the Godot API is documented in the class reference rather than C++ files. We can't afford copy-pasting all the method descriptions in the C++ source as they would quickly become outdated. For Godot core, the only way to avoid duplication would be to create an application that can read the C++ source code and generate the class reference with pre-filled descriptions based on that, but this is a lot of work for little gain.

Nonetheless, for godot-cpp's use cases, I wonder if it's possible to use the DocData available when generating the JSON API file which is then used to generate the bindings. Do note however that GDNative is being replaced with GDExtension in Godot 4.0, so this work would have to be redone from scratch there.

I asked a related question here: https://godotforums.org/d/36688-applying-the-class-reference-documentation-to-c

It is not always easy to figure out how to do something in godot-cpp after reading the godot class reference documentation. I give the example of the Transform3D * vector3 operation, which exists in the class reference but not in godot-cpp.

How do I, in the general case, find out how to implement, in C++, the features described in the class reference?

Now that PR godotengine/godot#82331 has been merged, we could have the code generator in godot-cpp add doc comments in a format that IDEs can understand (perhaps Doxygen?). Personally, I think this would be a great (optional) feature!