The problem is that they keep compiling the same version of Excel, just changing a little bit the color scheme each time.

Word, Excel, PowerPoint were all coded between 1989 and 1993. And received major update with Word 6 and Office 95 and 97. Basically Office 97 was feature complete. Very little has changed since then. The ribbon bar and all the other minor increment improvements are really minor compared to the changes up to Office 97. Office uses it's own UI library. Using Word 6 in Win95 shows floating toolbars in Win3.1 UI theme. Office 95 had an italic title bar. Using Office 97 in WinXP or Win7 shows and Win95 UI theme.

Well, MS removed quite a few ridiculous features from the Office 97 days. Like auto summary or Cliparts.

But removed some quite useful features at the same time, like the ability to customize the UI by moving the most frequently used buttons to the front.