Here's why:

* Composition: Oceanic crust is primarily composed of basalt and gabbro, which are denser than the granite and other rocks that make up continental crust.

* Thickness: Oceanic crust is thinner than continental crust, typically around 5-10 kilometers thick.

* Age: Oceanic crust is constantly being created at mid-ocean ridges and recycled back into the mantle at subduction zones. This means that it is generally younger and therefore denser than continental crust, which is much older.

While continental crust is thicker, it is less dense overall. This is why continental crust tends to "float" higher than oceanic crust, explaining why continents sit above sea level.