That's typically how software development works. If something is not being used in the game, it's a liability... a place for bugs to hide, and a drain on developer resources to keep that code tested when no one needs it. Just because they allow and encourage modding, doesn't mean they'll add engine features just for modders. Even extensively moddable games like Doom and Quake didn't do that. We get the same tools they get, and work within the same framework.
Did you ever
mod Quake? You had the full C source code for its main game logic (game.dll) available there ...
Newer Unreal engines have the ability to load and run additional DLLs written in C as well, with http://udn.epicgames.com/Three/DLLBind.html. Those can do nearly anything within the engine too, and anything you'd care about outside of it as well. No need for hacks like the script extenders, as much as I love the work people did there.