Because Emacs (and Vim, but when talking LISP, especially Emacs) is already magic. The visualization proposed is nifty, but it's hard to beat Emacs for any code editing, let alone for LISP.
Now I'm disturbed by the lack of parens. What about situations where I have one pair of parens exactly contained in another, like in a one-argument defmethod?
Edit: Now I see that I didn't notice that this situation happens in the "let maxcol 0" line in that image. I need to compare the colors on the line below to make sure I'm mentally parsing that line correctly.
Also, check this out: http://www.foldr.org/~michaelw/emacs/mwe-color-box.el (you can see a screenie in http://www.foldr.org/~michaelw/emacs/color-box.png)