Art and culture
Writers have rules that they believe are timeless rules of art, that are actually rules of their culture. The dominant paradigm in any culture says that art consists of those things that reinforce that culture's worldview.
The idea that stories must be based on a protagonist fixing an internal flaw may be based on a mistranslation of "hamartia" as "flaw" rather than as "mistake". The plays that Aristotle was talking about when he made that rule (e.g., Oedipus Rex) do not feature protagonists with fatal flaws; they feature protagonists who made mistakes. Oedipus Rex' mistakes were based on things he did not and could not know; so the entire story is not about a character learning and growing, but about the inevitability of fate. (Many Greek stories, such as the Iliad, do not fit Aristotle's model very well, suggesting that Aristotle was writing about how he wished tragedy to be rather than about how it was.)
The entire point of Beowulf (a feudalistic story) is that Beowulf does not change. Change, in those days, was bad. People were not supposed to change. They were supposed to meekly accept their role in life. It's no coincidence that Western literature as we know it today did not begin until the 14th century, and started to mature in the late 16th century. The stories we tell today would have been deemed dangerously subversive under feudalism.
(Exercise for the reader: Write an Aristotelian interpretation of the Who's musical Tommy.)