Well, there are many reasons, too numerous to list here.
So I will simply present Reason N+1. I had to upgrade from Office 2003 to Office 2007 for work. I've had it on my machine for maybe 20 minutes, and already I hate it.
Note the menu area. Note the huge clunky menu boxes.
I can't find a way to make them go away, other than an add-on I could get from Microsoft to revert to "Classic" toolbars. For $16.
So basically, huge bulky toolbox area with no way to revert to a more streamlined classic feel without paying more $$.
Edit, 9/18: Just found out I can make the bulky menu (aka "menu ribbon") go away - right-click on one of the ribbon tabs and select "Minimize This Ribbon". Much better! Thank you to those who pointed this out to me.
10 hours ago