The Tower of Babel explains why there exist multiple nations and languages across the Earth.
The Bible affirms that it was GOD who created multiple languages and spread out humanity into multiple nations around the world.
Following the great flood, we read:
1 Now the whole earth was one language and one speech.
2 And it came to pass, as they journeyed from the east, they found a plain in the land of Shinar, and dwelt there. 3 Then they said to one another, “Come, let us make bricks and bake them with fire.” They had brick for stone and asphalt for mortar. 4 They also said, “Come, let us build ourselves a city and a tower, whose top will reach to heaven; and let us make a name for ourselves, lest we be scattered abroad over the face of the whole earth.” 5 But the Lord came down to see the city and the tower the sons of men built. 6 Then the Lord said, “Indeed, the people are one race and one language, and they have begun to do what they said. Now they will not fail to accomplish what they have undertaken. 7 Come, let Us go down there and confuse their language, so they may not understand one another's speech.” 8 So the Lord scattered them abroad from there over the face of all the earth, and they ceased building the city and the tower. 9 Therefore its name is called Babel, because there the Lord confused the languages of all the earth; and from there the Lord God scattered them abroad over the face of all the earth.
(Genesis 11:1-9 OSB)
In the first century A.D., the apostle Paul proclaimed to the men of Athens, Greece:
24 The God who made the world and everything that is in it, since He is Lord of heaven and earth, does not dwell in temples made by hands; 25 nor is He served by human hands, as though He needed anything, since He Himself gives to all people life and breath and all things; 26 and He made from one man every nation of mankind to live on all the face of the earth, having determined their appointed times and the boundaries of their habitation, 27 that they would seek God, if perhaps they might feel around for Him and find Him, though He is not far from each one of us; 28 for in Him we live and move and exist, as even some of your own poets have said, ‘For we also are His descendants.’
(Acts 17:24-28 NASB)
The origin of humanity's diverse cultures—at least those that exist today, following the flood—was not gradual, but the immediate result of the events at Babel.
GOD has predetermined each and every nation's existence, duration, as well as its territory.