The reason why the author of On the Sublime (who more than likely is not Longinus, according to historians) sheds a light on the production itself rather than the poet is because of a term that the author quoted from Theodorus which is parenthyrsus (III). This term refers to a tragic flaw in all men which is to lose themselves into a situation. In the words of the author,
men are often carried away, as if by intoxication, into displays of emotion which are not caused by the nature of the subject, but are purely personal and wearisome. In consequence they seem to hearers who are in no wise affected to act in an ungainly way. And no wonder; for they are beside themselves, while their hearers are not.
If we combine this statement with the meaning that the author gives to the sublime, we can conclude that, in the opinion of the author, man himself does not have the capacity of causing the astonishment. The production that comes out of the flawed man, poet or not, is what ultimately produces the transcendental effect that is hoped to be elicited in the hearer.
The other reason is that the production is meant to be a gift that keeps on giving, it is something metaphysical that affects everyone in a different way, bringing up pent up emotions, or inspiring emotions unknown.
For, as if instinctively, our soul is uplifted by the true sublime; it takes a proud flight, and is filled with joy and vaunting, as though it had itself produced what it has heard.
To shed light on the poet alone would mean that the poet was filled with joy and vaunting, and the result was the production of a poem. Yet, the metaphysical trait that the author conveys upon the production, entails that the production will inspire both, the poet and the listener. The production, once out of the mouth of the poet, takes a life of its own and moves on to astonish the lives of others. That is the basic premise of what "sublime" is according to the essay.