The issue is more complex, friends.
I’m going to leave this link here because I think it explains much better than me the situation of several peoples/kingdoms in the world (although the focus is on Africa) with respect to our knowledge of their history.
But to summarize, the point is that there is enough information about these people/places to satisfy the (few) demands of the game, with the biggest difficulty being the issue of access to this information. However, this is slowly being resolved by both increased study of these people/places and exposure in various types of media about their history (breaking the feedback loop of ignorance).
I say this because I, a total layman, was able to gain reasonable knowledge just by using the web. This list I made a while ago brings together sources on the history of Africa (over time, I expanded it without much organization and I intend to include other areas of the world as well). If you look, you’ll notice that many of the articles and even some books are freely accessible and for those that aren’t, you can see summaries on sites like r/askhistorians and historum.com. (Just these FAQs here and here would be of great use.)