Note that Emacs and GNU Emacs are different things. Emacs is a family of editors, which share some basic design principles (buffers, keymaps, modes, extended commands, ...).
Why do you think Lisp IDEs look like some Emacs? Because you happen to know them and haven't seen the other ones.
Many Lisp IDE's have an Emacs editor, but are not based on Emacs. For example the MIT Lisp Machine has Zmacs, but that's mostly used for editing text (and some functionality around it). Zmacs is also used as a component in a few other programs, like the Mail client. Other than that most Lisp Machine programs are neither based on Zmacs nor have a Zmacs UI. Examples: file browser, font editor, terminal, Lisp repl - none of that feels/looks like Emacs.
Interlisp-D / Medley from Xerox didn't look like Emacs at all.
The Allegro CL IDE doesn't look like Emacs.
LispWorks uses an Emacs written in Common Lisp, but for example does not share the buffer handling (in LispWorks buffers have their own windows all the time and they can't be changed). Additionally much of the LispWorks IDE is not based on an editor paradigm (inspectors, debugger, preference windows, ...).
Since 15 years most (not all) open source CL developers used GNU Emacs + SLIME as the IDE. SLIME replaced older tools for GNU Emacs. There wasn't much competition for SLIME - it also was relatively easy for Lisp programmers to add IDE features to an editor extensible in some Lisp dialect - it's much more difficult and more difficult to make Eclipse or Intellij a decent Lisp IDE, and the results so far are clunky. SLIME OTOH is quite usable as a Lisp IDE.
It would be about time to have a better Lisp IDE than GNU Emacs / SLIME - especially one which is more graphical. But then, we now have the javascript web and GUIs are again different.
I was referring to recent-ish IDEs, although from all those you mentioned the only that do not look like Emacs to me are Allegro CL and Xerox's. The rest might not have exactly the same UI, but they still have a UI that looks very similar to Emacs.
As an example to the contrary, both the Allegro CL and DrRacket would be UIs that are not similar Emacs.
Because Lisp programmers like to extend their tools using Lisp. Which happens to be the case because Lisp is the most powerful language to have ever existed.
When you use Lisp to make an IDE to edit Lisp. Emacs like software is what you get.
For a programmers' text editor, you could do a lot worse than starting with the Emacs UI. (Even if working on character cell terminals, and being familiar to users, are not requirements.)
We should always be experimenting and finding better ways to do things, but not every such project has to start with Rethinking the Modeline and Echo/Input Area. :)
