Here's why:

* Accessibility: The crust is the outermost layer and the only layer we can physically access through mining, drilling, and surface exploration.

* Direct Observation: Geologists can directly observe rocks, minerals, and geological structures within the crust.

* Indirect Evidence: While we can't directly access the mantle and core, geologists use indirect methods like seismic waves, magnetic field measurements, and gravitational studies to learn about their composition and behavior.

Let me know if you'd like more details about how geologists study the Earth's crust!