More to the point, Xcode, Visual Studio, Android Studio, IntelliJ, Eclipse, and so forth all have generally different UIs, which means that unless you spend all your development time in a single environment, you have to do a lot of really expensive mental context switching.
An editor like Emacs or Vim provides a unified user interface for all the different languages and development environments with which you use it, which eliminates almost all of that mental overhead - and can also invoke your compiler, (usually) integrate with your debugger, et cetera. IDEs still win on tight integration, but for any language popular enough that someone's invested the time to build a well-integrated IDE for it, Emacs or Vim will generally cover at least the 90% cases quite well, too.
An editor like Emacs or Vim provides a unified user interface for all the different languages and development environments with which you use it, which eliminates almost all of that mental overhead - and can also invoke your compiler, (usually) integrate with your debugger, et cetera. IDEs still win on tight integration, but for any language popular enough that someone's invested the time to build a well-integrated IDE for it, Emacs or Vim will generally cover at least the 90% cases quite well, too.