Because it’s a bad take and conveys a shallow, simplistic understanding of literature as either fact or myth. Viewing literary artifacts in such a way is too black and white, overly modern and just not very useful when it comes to serious study.
How exactly should literature be viewed then? I am having difficulty understanding what exactly your complaint is about the objective filters about something being fact or myth when it's (by definition) an unambiguous state of being for most ideas and legends, literary or otherwise. How does talking about it damage "serious study"?
