Why is `C-M-x' only for top-level defuns?

[Drew Adams]

There is likely a good reason for this, but I'll still pose the question, to
learn.

Why not let `C-M-x' re-evaluate a "defun" (defcustom, defface, etc.) that is not
necessarily at top level?  E.g., with point on, say, `defface' in this sexp, why
shouldn't `C-M-x' redefine the face?

(when twillig-&-the-tithy-toves-do-tyre-&-timble-in-the-twabe
  (defface ...))

Currently, in order to make `C-M-x' work for such a "defun" I temporarily move
its first line to column 1 and then hit `C-M-x'.  Seems like a silly workaround,
but I don't know of a better one.  Am I missing something?