Unity of key bindings is one reason. Consistency of interface. Relative ease of using elisp to write some of these things.

Emacs users typically have a whole pile of customizations, macros, and workflow automations they've written up over the years.

And for things that are text oriented, the ability to work the buffers emacs-style between emacs windows is a joy.

There's a lot of advantages to keeping everything inside emacs.