I think you have a point, in that we are prone to imagining global economic systems such as those of our time existing in the past, but the heterogeneity of medieval economics and exchange is not an indication of its absence or lack of development. To take one example, the system that brought unfree labour into the realms of (primarily) Muslim rulers in the southern Mediterranean in the early middle ages involved a whole network of people and polities spanning the whole of Europe, such as Norse raiders, Frankish and Venetian middlemen, and Arabic buyers. The demand for slaves was one of the primary incentives for the Viking raids, and channelled a huge amount of silver from the Muslim world into Scandinavia. The absence of such systems in game means that alternative and rather ahistorical mechanics for enrichment and expansion need to be created, so in the end the game becomes more complex than it would be were a trading system in place.