Yes, emacs is wonderfully customizable, but there's a deeper idea underneath. By investing time and effort into learning it and buying into its conceptual view of the world as text buffers that can be manipulated, you gain a level of mastery over your environment that's simply not possible with an IDE or specialist tool. And we are all deeply motivated by mastery. Some authors like Dan Pink say that mastering something is the biggest motivation of all, even above recognition and money.

Many IDEs are pretty veneers on sequences of complex commands that must be learned by rote. Emacs gives you text buffer interfaces to whatever you want to do, a mostly consistent model for dealing with them and the programmability to change whatever you like.