Because it’s readily available even on the target machines.
IDEs are good (for some; I’m not one of them). Besides code editing, which is poorly supported in many IDEs, they offer a lot of other functionality like building/compiling, editing a user interface file, and so on.
Yet, I use Emacs. I simply like it. I worked with Vim for about 15 years, and switched to Emacs a few years ago. But as my projects vary a lot in language and framework, I can’t really stick to an IDE.
I do have Android Studio on my machine, though. I build my projects with that, because I didn’t have the time to learn how to do it yet. But I edit all the Kotlin files in Emacs, because Android Studio doesn’t understand C-x C-s. So yeah, muscle memory, too.
At the end I can’t say ViM/Emacs is better than any IDE. What is more important: choose a tool, stick to it, and learn it well. It will become an extension to your arms soon, and that is what makes you productive.