To help implement these types of features the IDE embeds a version of the language inside of itself.
The problem with this is you then need a new IDE for each new language.
Because a good programmer's editor will try to be language neutral it does not have that luxury.
But as a bonus, because the editor is not carrying around all that extra language baggage the editor will always be faster than the IDE.
Well, it would be nice if we had some "IDE framework" to take care of all the barebones stuff (editor pane, basic UI, loading files, encoding, high level abstract support for highlighting, refactoring, embedding a terminal, etc) and people could hook it into their language to make an IDE for it.
And it would have to be written in C, to not usable from all languages with a C ABI/FFI, and not be dog slow like Eclipse.
I was describing a "toolkit" like approach, not an extensible base. Think of it as a "set of libraries" vs a "framework".
Emacs makes what I describe difficult. You cannot change the editor. You cannot add a different GUI on top. You have to do most of the extension work in elisp, etc.
I would prefer a set of libs a Ruby guy can hook up with Ruby and extend, a Python guy with Python, etc. Not a one-size-plus-extensions-fits-all.