Civ is, was, and will always be strange and inconsistent in this regard. There also is no 'Aztec', but the civ is named like that. I thought they just did it do not have a split again between countries/empires and peoples. Where they found Sumeria, I don't know. Maybe they also wanted to avoid the people as a name (Sumerians) and ended with Sumeria instead of Sumer for some reason? Since I'll be playing the translated german version, I wonder how they'll translate those two, maybe the translators will use the peoples again.