First of all "Go" is not a sentence because it is not a complete thought or idea. In order to use "Go" as a command it has to be supported by previous conversation so that the person being commanded will understand that they are the person being spoken to. Likewise the command needs to have a coinciding gesture such as a look or pointing a finger in order that the person being talked to knows that they are the one being commanded to do something. Another way to look at it is to put the word "Go" on a piece of paper. If you look at the word "Go" it does not tell who to go or where to go. Therefore it is the physical gesture you use along with the command "Go" that tells the person what to do not simply the command "Go" by itself. "I am" on the other hand does not need a physical gesture in order to be understood. Likewise you can transfer these words to paper and the statement is completely understood and consists of a subject and a verb. Debate closed.