I think it is because there was no inspirational leader to actually push for building something greater. Generals rushing to fill a power vacuum is different from a visionary who rose to their position by the will of the people. In the real world, there are vast regions of the planet that were populated in ancient days but are allowed to go unused and fall into disrepair because there is no vision that prompts settlers to reclaim them.

Think of this: the only Westlanders that seems to want to see the rest of the world are Jain Farstrider and travelling merchants. Noone else, even the Seafolk, want to venture beyond what is familiar because they work so hard to keep what they have.

As hard as it is to envision, given the history we are told prior to the story's opening, from the Trolloc Wars to Hawkwings' time to the current Age, the planet of WoT has been on a slow slide downwards from the Breaking. Though we see the current cultures as being in a rather good position, they are still just survivors. At the end of the day, Hawkwing just united a small group of nations on the fringe of a vast continent, with his son doing slightly better. We've never heard of anyone going to the other side of the planet or even exploring the poles - it's all wide open to discovery and reclamation, but everyone is so caught up in the equivalent of small-town politics that a grand vision hasn't really had room to be nurtured.