Here's why:

* Continental Plates: Some plates are primarily made up of continental crust, which is thicker and less dense than oceanic crust. Examples include the North American Plate, the Eurasian Plate, and the African Plate.

* Oceanic Plates: Other plates are primarily made up of oceanic crust, which is thinner and denser than continental crust. Examples include the Pacific Plate and the Nazca Plate.

* Mixed Plates: Some plates are a combination of both continental and oceanic crust. For example, the South American Plate contains both the South American continent and a portion of the Atlantic Ocean floor.

In summary: While oceanic crust is a fundamental part of the Earth's tectonic system, not all tectonic plates are entirely composed of it.