Other than what people have said before, I don't think Firaxis is placing civs specifically for other civs to have easier paths (otherwise Hawaii and Inca would not be in base game). They're targeting civs specifically, and classifying them roughly with the civ's timeline and cultural role.
This is Firaxis's reasoning for where each civ goes where (the comment is specifically why Khmer is in Antiquity), and I think Antiquity fits Silla most here.
Great comment, u/jalapisa! On identifying Ages, we sought to capture and represent general historical trends that were happening roughly around the same time period. One thing we didn't want to do was have the events of the Mediterranean dictate a calendar for the rest of the world. So if we were to summarize some general processes within each Age:
Antiquity is characterized by competition between states and non-state regions around them – the “blank spaces” on the map. It is a time of city-building, of universalism and expansion, where states claim a mantle of absolute authority. This is the time when states claim to represent the heavens, and that their language is the one true one.
Exploration is a time of vernacularization – when these prior empires split into fragments of the former whole, and where local innovations alter what was there before. It is a time when universal religions rise to suture this gap, but where interconnections – especially global interconnections – come to define states.
Modernity is a retrenchment of empire. Here, modern and scientific thought, bureaucracy, has replaced or fused with notions of divine right, and empires are increasingly seeking to understand, catalog, control, and apportion their subjects.
In that way, Khmer was a better fit for Antiquity – early Khmer was continually expanding into non-state lands, the building and establishment of cities and the construction of a mandala state - a center-oriented city that sought to bring the cosmos into orbit around itself. In creating this gravitational/civilizational pull, Khmer cast itself as a universal center for civilization – something which resonates much more with Antiquity states elsewhere.
Importantly, there are also excellent descendants in the region that are doing very Exploration Age related things - so having Khmer in Antiquity allows us to create a more solid throughline for Southeast Asia.
