What the hell is going on in the rest of Earwa? This is only one continent or even just part of a continent. Was it mentioned anywhere this is the only land mass on the planet? Me thinks it would be wild to have a completely different story line for "everywhere else", so see isolated groups of life evolving separably, could really show off some of Bakker's muscle. The Non-men seemed pretty powerful, intelligent and advanced - no explorers among them? Or did they all think the world was flat ...
The Nonmen were largely not interested in ocean travel, apart from those of Illisseru:
(from TUC glossary, absolutely no spoilers for the plot but some new backstory information)
Whom the other Nonmen thought were insane and then proved it by pretty much wiping themselves out in the Cuno-Inchoroi Wars with a grand naval assault on Golgotterath that went about as well as could be expected.
More to the point, the Nonmen birth rate was very low and they expanded across Earwa very slowly, establishing only nine large Mansions (populations in the tens to low hundreds of thousands) over 10,000-20,000 odd years. Without population pressures and with apparently no other civilisation elsewhere on the planet, they was no economic or population pressure to expand or explore the rest of the planet at that time.
For the present era, it sounds like the terrain on the far side of the Kayarsus is not particularly pleasant and there is no real impetus to explore eastwards across the rest of the continent. If there was a powerful kingdom or empire out there, you'd assume it would have made itself known. As such, civilisation in Eanna is probably far less sophisticated than in Earwa. There's been no reliable intercontinental contact for 2,000+ years, since the Xiuhianni attacked Jekk over the South Kayarsus, which might have been the last gasp of a dying or less sophisticated race.
Zeum apparently has superior naval technology to the Three Seas and may have explored further into the ocean, but if they've found anything, they've not told anyone else. They may have explored the coast of Kutnarmu as well, but I think we can assume that northern Kutnarmu is Africa 2.0 and just uninhabitable desert apart from Cingulat.
More simply, Earwa is Europe somewhere between Ancient Greece and the Crusades, during which time exploring for exploration's sake really didn't happen, and it took 2,000 years from making the first reliable maps of the North Africa coast and finally founding the far southern tip.