The Emacs API is kinda huge, with things like very raw network API, a very good approximation of fork/exec process management, buffers as the base communication mechanism with a lot of capabilities, various utility function with regards to interfacing with the user (windows, frame, faces, keyboard events), then the hooks and advice subsystem.

With Unix, most programs are binary and while the shell is a good glue language, you can’t alter a program and the OS that much. With Emacs, only the core coded in C is sacred, anything else can be modified to fit your workflow. And there’s a lot of packages out there to provide you with raw materials.