We know by the number of real kings who claimed descent from Gilgamesh that there were enough people who believed this that it was worth it politically. Same as

We know by the number of real kings who claimed descent from Gilgamesh that there were enough people who believed this that it was worth it politically. Same as we know enough people in medieval Europe believed in the literal truth of Christianity to make it worth for kings to claim to be chosen by God for their purpose. Contrast this to the lack of modern day leaders claiming descent from Batman, or claiming the legitimacy of the Ministry of Magic. Of course, there would have certainly been people who did not believe in the literal truth of these stories - either as some form of early naturalism, or simply because they believed in other stories (like how devout Christians view the Bible as literal truth, but think the Veddas are just stories, and vice versa).