Here's why:
* Dictionaries are alphabetical: Words are listed in alphabetical order, so the "last word" would depend on the exact edition and how the dictionary handles things like proper nouns, foreign words, etc.
* Dictionaries are always being updated: New words are added, definitions change, and words might be removed. So, the "last word" wouldn't be fixed.
The fun part of this question is that it makes you think about how dictionaries are structured and how language is constantly evolving!