Just a guess, but I think it's because the message should indicate what the commit itself does, not what _you_ did to write it. For example, by merging this commit, the result is that I "Add X widget" to my code.
This is exactly how I describe this idea to people. I tell them to complete the following sentence. "Applying this change set will...". I've recently become less averse to verbs like fixes, updates adds, etc as I feel the sentence "The change set..." is still pretty good. The key, obviously, being you're describing the change set, not your actions.