Here's why:

* Supercontinent Gondwana: Southern continents (Africa, South America, Antarctica, Australia, and India) were once part of a supercontinent called Gondwana.

* Ancient Mountain Building: Gondwana experienced periods of intense mountain building, which formed large granite batholiths (massive intrusions of igneous rock).

* Continental Drift: As Gondwana broke apart, these granites were distributed across the continents.

While other rock types are also present, granite is a distinctive and widespread marker of the ancient Gondwanan history.