Genertela was in a true Dark Age at the start of the Third Age. Between the Closing of the Seas and the Dragonkill War, trade (and thus urban civilization) was minimal. Ralios and the Holy Country had contact through the Manirian Road – a series of caravans that went from Esrolia to Safelster and Tanisor. Peloria was cut off from the south and Ralios, and had only the long river route down the Janube to Sog City. This route was controlled by the Carmanian Empire, but its long distance made it very expensive and slow.
Although Dragon Pass was open during Sheng Seleris era, there was little trade. The founding of the Kingdom of Sartar in 1492 changed things – a high volume of trade could pass between the Holy Country and the Lunar Empire, a volume that only increased as the Sartar Dynasty built roads, inns, and otherwise encouraged and protected that traffic. The Holy Country and Sartar grew wealthy hand in hand, and trade and cultural connections grew even stronger.When the Opening took place in the 1580s, this became supercharged. Trade with the whole world became possible through the Holy Country, and spices, herbs, silks, and other exotic luxuries came across Dragon Pass into the Lunar Empire. The Lunar nobility greedily bought whatever luxuries they could, and coveted the great riches they brought the rulers of Sartar and the Holy Country.
Remember, the Third Age where the Red Goddess or Sartar walked the earth was a very different place than the later Third Age of the Hero Wars.
And also that the pre-Sheng Seleris Lunar Empire was a very different place from the modern Lunar Empire. Think how different the United States of 2021 is from the United States when James Monroe was president. A few common documents, titles, and symbols, and some shared place names. But socially, economically, and politically completely different.