When the people talk of this world’s history, they speak of things that happened hundreds and even thousands of years ago, yet apparently their culture and technology have not changed at all.
Why this can be bad: It doesn’t make sense. Cultures evolve. Science evolves. So how has this world not? There are sometimes centuries where a culture or part of the world doesn’t advance, or even regresses (hello, Dark Ages), but these don’t last millennia. Humans are constantly changing, finding new ways to fix old problems, and learning. The idea that a culture would go five centuries without a single significant advancement is unbelievable. This is one of my few criticisms of A Song of Ice and Fire: a written language has existed a thousand years ago because there are actual thousand-year-old books, but yet somehow the known world’s culture never progressed farther than that. Not their technology, not their culture, and definitely not their politics.
How you can fix it: If you want your culture to have millennium-old stories, turn them into folklore or legend. These are passed down and exaggerated, so any discrepancies in time make sense. You can also use this to create more world-building for your story. When you’re creating your world’s history, you have to take into account what is realistic. Look at timelines that outline the evolution of cultures. Research how the facets of a society interact to stagnate or encourage growth.
Bottom Line: Even fictional worlds need to mirror the real one.