Here's why:

* Divergent boundaries: These are areas where tectonic plates move apart. As they pull away, magma from the Earth's mantle rises to the surface, cools, and solidifies, creating new oceanic crust. This process is known as seafloor spreading.

* Convergent boundaries: At convergent boundaries, plates collide. When an oceanic plate collides with another oceanic plate, one is subducted (forced under) the other. This process destroys oceanic crust, not creates it.

* Transform boundaries: These boundaries involve plates sliding past each other horizontally. They don't create or destroy oceanic crust.

In summary: Oceanic crust is primarily formed at divergent plate boundaries, where seafloor spreading occurs.