The game is set in a post-apocalyptic utopia where you rebuild society after a global collapse. A massive online multiplayer where everyone plays on an exact replica of planet earth. Grow your town, trade with other players, face challenges and discover the rich history of the planet.
But those 81 cities take up a tiny portion of global land area.
A quick Excel import of the table from Wikipedia suggests that the “urban area” for those 81 cities is 190 K KM(2).
Total land mass on early 148.94 M KM(2).
So the world land could hold ~63,400 such cities.
It would likely take many years to build a city like Guadalajara (#81 on the wikipedia list).