A small child turns to Ed, and exclaims: "Look! Look! A post from jon
<jon@jonhester.com>!"...

Because the people who wrote Publisher didn't map the F12 key to the Save As
dialog?

Or do you mean why did the programmers not do that?

At the moment... we just don't know

This group is about programming with the Publisher Object Model.  General
questions about Publisher belong in the microsoft.public.publisher
newsgroup.

There is a Publisher add-in available that called "Shortcut Manager for
Publisher" that allows you to define as many keyboard shortcuts as you want.
Unfortunately, it costs $25 - it's available from
http://officeone.mvps.org/pbsctmgr/pbsctmgr.html