Here's why:

* Seafloor spreading: New oceanic crust is constantly being generated at mid-ocean ridges, pushing older crust away. This process, known as seafloor spreading, means that the oldest seafloor rocks are found farthest from the ridges.

* Continental stability: Continental crust is much older and thicker than oceanic crust. It's less prone to subduction (being pulled back into the mantle) and is more stable.

* Recycling: Oceanic crust is constantly recycled through subduction, where it's pushed back into the mantle. This means that the oldest seafloor rocks are relatively young compared to continental rocks.

In summary: While there are exceptions, the majority of seafloor rocks are younger than continental rocks due to the continuous cycle of seafloor spreading and subduction.