Here's why:

* Gabbro is a mafic igneous rock, meaning it is rich in magnesium and iron, and has a relatively low silica content (45-55%).

* Granite is a felsic igneous rock, meaning it is rich in feldspar and silica, and has a high silica content (65-75%).

This difference in silica content results in distinct physical properties:

* Gabbro is typically darker in color and denser than granite.

* Granite is typically lighter in color and less dense than gabbro.

So, while both are igneous rocks, their silica content and resulting characteristics are quite different.