Howso?
In FFXIV, for example, we do not have access to the "traditional" or "original" culture of several groups. Sometimes, it is because those cultures no longer exist, e.g. the modern city of Ul'dah was founded by a lalafell (halfling) majority culture called Belah'dia, but Belah'dia ceased to exist over a thousand years ago and all we have left are some ruins of their religious and political structures. The modern culture of Ul'dah features mostly hyur (humans), lalafells, and roegadyn (effectively orcs), while having relatively few elezen (elves) and miqo'te ("catgirl/catboy" types, as opposed to Khajiit-like humanoid cats.) Similarly, lalafells in general originally come from the south sea isles, demonstrating a form of insular dwarfism most likely, but the south sea isles are not accessible to players so we don't know what their culture is like in specific. As a result, we know that there are and have been multiple lalafell-majority cultures, and that lalafells exist in many cultures (though not all cultures) of the present day, without actually knowing much if anything about any "original" culture they may have possessed.
I don't see why it cannot be the same for a D&D game. It might be the case that there was in fact one single ancestral culture from which all modern cultures descend, but that culture may be inaccessible by reasons of time or geography.
