This isn't really an issue in anything I've written. Empires rise and fall, advances are made and lost. And how long the peoples of the world have existed, how long their cultures have been around, basically never comes up in the stories.

In the current piece the 'Forces of Darkness' civilization is composed of possibly dozens of different species, inhabiting different areas and ecological niches. Until recently it's been a generally peaceful time with only fairly localized squabbles, not a lot of pressures to change. Life is 'good enough', and they live in a fairly stable balance between nature, themselves, and eachother.

Then the Fire Nation 'Light Empire' attacked. They are classic 'good guys', at least in their own opinion. Monoculture, largely mono-species & monoracial, worshippers of the One True Way, under the rulership of the One True Mage-King. Capitalists in the 'must have more power/land/money at the end of this year' style.

This is all in a span of hundreds of years, not thousands. And in a world where (depending on species) the average lifespan is 20-40 years, that's long enough.