Here's why:

* Continental crust is older: Continental crust is much older than oceanic crust, which is constantly being created and destroyed at plate boundaries. Continental crust is more stable and less prone to being subducted back into the Earth's mantle.

* Ancient rock formations: Some of the oldest rocks found on Earth are within continental shields, which are vast areas of ancient, stable continental crust. These shields often contain rocks that formed during the Archean Eon, which spans from 4 to 2.5 billion years ago.

Examples of very old rocks:

* Acasta Gneiss, Canada: This rock formation is estimated to be around 4.03 billion years old, making it one of the oldest known rocks on Earth.

* Isua Greenstone Belt, Greenland: This belt contains rocks that date back to 3.7 billion years ago, providing valuable insights into early Earth history.

While continental crust can be very old, it's important to note that not all rocks in continental crust are billions of years old. There are also younger rocks within continental crust, formed through geological processes like volcanism, sedimentation, and metamorphism.