Here's why:

* Oceanic lithosphere is primarily composed of basalt and gabbro, which are mafic rocks. These rocks form from the cooling and solidification of magma erupted from mid-ocean ridges.

* Continental lithosphere is more diverse, containing a wider range of rock types including felsic and intermediate rocks like granite, diorite, and andesite. While continental lithosphere does contain some mafic rocks, they are not as prevalent as in oceanic lithosphere.

Therefore, oceanic lithosphere is significantly more mafic than continental lithosphere.