Eclipse seemed to me to be a "substrate for Eclipse plugins" first and foremost, whereas both NetBeans and IntelliJ struck me as "tools for writing code" first.

That's how it was designed, from even back when it was VisualAge and written in SmallTalk.

Eclipse is basically a language-neutral UI "shell", the "Eclipse Tool Platform", where all language/code-related functionality is provided as bundles of extensions. And then extensions can be extended, and so on.

Eventually they standardized the extension system around an extension/component framework called OSGi. Not sure if this new Eclipse IDE uses it.