Of course there won't be Incas on release at this point. Do you think they would drop Incas in the medieval era without any ancestors for them to grow from, now that we see how important "lineages" seem to be for their culture selection?
You can't symmetrically cover North America, Central America, South America, West Africa, South Africa, East Africa, North Africa, Western Europe, Southern Europe, Eastern Europe, Middle East, Central Asia, East Asia, South Asia and South East Asia... In release version of a game, when there are only 10 cultures per era - and regions have very unequal distribution so some get a lot of cultures in the same era.