You can see this with UnrealScript for example.

It's pretty clear why this is done, most game designers aren't experienced programmers and there is a very large push to simplify game development in both the code that needs to be written and allowing a lot of development to be done with visual editors.

If you want to add a new shader it's much easier to develop a shader generator and expose your internal engine functionality via DSL or a UI elements than teach everyone on the dev team how to write shaders, especially since your engine might need to support multiple platforms, so you have a compiler that can target different hardware/software platforms.

Same goes for every other aspect of game development like game mechanics, AI and physics.

So you end up having a small core team of engine and tool programmers and then 100's of game designers that can then work on a higher level.

And it's not that games are unique, this is how computer programming always worked, the core of C e.g. something like printf has to be written in assembly, same goes for your core C compiler (yes now you can technically write a compiler for C in any language including C but when you start from scratch you need to bootstrap it with something).

In fact this is how the web itself works, we have a core "engine" - the browser and DSL - JavaScript and HTML.