Emacs and vi(m) are great editors today, but imagine how much even greater they would be if all those people pouring hundreds and thousands of man-hours into projects like Gedit, JEdit, KWrite, Medit, etc. would have instead put their efforts towards improving the true editors!
Disclaimer: I haven't really used those other editors a lot and I'm sure that each of them has something going for it.
Also, I know how much fun it is to work on something challenging, like a text editor, even if (or exactly because) there are already established "solutions".
Finally, a few alternative editors are genuinely useful, such as nano - it might be inferior in pretty much every regard to Emacs, but it's not possible to shrink Emacs to nearly the size of nano so it does seem to serve a valuable purpose for when disk space or memory footprint is an issue. Cue the Eight Megabytes And Constantly Swapping jokes.
EDIT: and yes, Emacs/vi(m) are not necessarily the be-all, end-all of editors for all time to come; maybe one of those alternative editors will one day prove to be superior. All those other editors just remind me of the phrase "Those who don't understand Emacs/vi are condemned to reinvent it, poorly"... err, or something like that.