Why Do GUI's Look the Same?

MaxVlast asks: "I was browsing around Helix Code's site looking at their interpretation of GNOME when I found the program that they claim is 'the next step forward in GNOME applications,' Evolution. I was startled and upset--this program is, from what I can tell, a direct transfer of MS Outlook to Linux. It's bothered me for some time that the two major file and desktop managers for Linux are all clamoring to look more like Windows than their competitors, and in the scuffle, are missing some very effective paradigms (like Miller columns). Do people think it is good that Linux seems to be shooting for the 'looks like Windows, but without all the features' market? The popularly available apps seem to say so." Please pause for a moment before saying "But you've done this already!". Before the question was "Do new GUIs exist?", the answer to that was a resounding yes, but now the question is "Why do our applications still look the same?", even across platform boundaries and for two different applications?

Why does Evolution have to look like Microsoft Office? I'll buy the form-follows-function argument to an extent, but there are other alternatives to things like the standard menu/button bar and other GUI elements that could be applied to applications and at the very least, give users more choice in how they operate. Are there any projects looking to bring other not-often-seen UI elements like pie-menus and the previously mentioned Miller Columns ^[?] to our applications?