One of the reasons I haven't seen mentioned much is: I'm a touch typist, and I hate having to switch between mouse and keyboard! I have to take my attention off the screen. Slows me down. Disturbs my concentration.
With Emacs, I can work for hours without ever having to hunt for the mouse, or without even having a mouse plugged in. That's particularly nice for using my Ubuntu laptop on a lap desk, leaning back in my armchair. The best mouse is no mouse at all.
Yes, I know many IDEs have shortcut keys for some commands, but the basic assumption with GUI applications is that you have a mouse. Even if you learn the shortcuts, somehow you always end up having to use the mouse for some common task, e.g., to select a paragraph. That's why I never got to like a GUI IDE, despite having had to use Eclipse for years.