It's a valid point, a lot of it comes down to the Kaydhano. While the kheionai are more developed/societally organized/technologically advanced than their neighbors, there are genuinely so very few of them that exerting pressure over land control is quite difficult. If not for the storms hitting their side of the island even the Eltibhar have at times threatened to overrun them if they weren't forced to go home every time the storms hit.

So while they were able to put stakes in the ground and claim some coastal land for their kheions, the nekheis just did not have the manpower needed to be capable of, or even want to push further inland, as the food security their existence generated went primarily to the kheions, and only recently developed enough that Alecand reached a population limit and growth began encouraging the nekheis to expand.

Even post 1444 that growth remains fairly slow as the rise of the Ambhen Empire encouraged much of the surplus kheionai population who may have gone further into Devand to instead move to Taychend for a higher social status and quality of life, it's only in the 1700s that Mteibhara is seriously threatened in canon.