If you ask Microsoft Word to open two identically-named files (stored in different directories), it happily obliges. However, if you try the same trick with its sibling Office application Excel, you get this error message (click for the full version):
While the message explains both possible workarounds (close or rename one of the files), it doesn't offer any insight why the restriction exists in the first place. However, the official Microsoft Excel Blog offers up an explanation: Excel formulas can make reference to cells in other open files via their simple filename, so with two identically-named files open there'd be no way to tell which ones was needed:
If you had a cell ='[Book1.xlsx] Sheet1'!$G$33 and you had two books named "Book1" open, there's no way to tell which one you mean.