I've used emacs for about 10 years (written plugins, declared emacs.d bankruptcy, gone all-in on org-mode), so I'm familiar with the pros of configurable editors.

Here's a list of more advanced functionality built into Intellij that would be difficult to replicate:

- Bullet-proof navigation and advanced refactoring that works every time across a wide variety of languages.

- Database integration: syntax highlighting for SQL code in strings, runnable sql, database introspection

- Debugging: consistency and polish across languages, not relying on external tools that might not be installed, visual debugger

Text-editors struggle with the "integrated" aspect of an IDE. It's not terribly difficult to coerce text editors to perform well for a single language. It's much harder to integrate plugins and provide a consistent experience.

There's good reasons to use text editors (learn how it all works, extreme customization, magit, lightweight) but there's many development tasks Intellij is just better at.