The world changed a lot in the years following after Shiba and Eini returned to the blessed island.
Back when they roamed the world, the war was spread on a lot of counties due to the aftereffects of the Holy War, but it had already calmed down after so many years.
Most counties disappeared without leaving a trace behind due to their internal and external struggles, and the remaining ones merged together. Thus, after the turbulent period the world had spent, only a single county remained. The whole world had been unified.
All the Rank 9 survivors from the Holy War decided that it would be the best way to control the planet and avoid that something like the Holy War happened again.
With all the Rank 9 experts working together, there was no way they didn't succeed since Shiba's group had been isolated from the rest of the world on the blessed island.
The capital wasn't set on any of the old capitals but on a reconstructed city that was just named the World's Capital.
