Parentheses make perfect sense in nonfiction writing -- I, author, am telling you that the EI is not very important to whatever I am trying to say. In fact, you can skip the EI and still understand the main point. It's just an informational aside.

In fiction writing, parentheses tend to be hypocritical. First, is there really some main point? No, there is just a story. Second, if the EI isn't important to the story, why is the narrator presenting it?

Third, there is no way of speaking that actually corresponds to parentheses (as far as I know). There is a sense that the narrator is talking to the reader, especially in first person, so parentheses in a way don't make sense.

So, actually, parentheses aren't used that much in fiction writing.