"What is literature?" is often just a dominance game, I think. The predefined answer is: "Why, it's my interests and area of expertise—not yours."

If someone truly believed in objective metrics for "literature," like a five-act narrative structure or at least five percent of words having five syllables, then they could just say that. But probably they won't box themselves into such a definition because they want to reserve the right to accommodate whatever books may interest them in the future.