Here's why:

* Lithosphere: This is the rigid outermost layer of the Earth, made up of the crust and the uppermost part of the mantle. The lithosphere is broken into large pieces called tectonic plates.

* Asthenosphere: Beneath the lithosphere lies the asthenosphere, which is a semi-molten layer of the upper mantle. The asthenosphere is not rigid, allowing the tectonic plates to move and interact.

It's important to remember that the Earth's structure is divided into layers based on their physical properties (solid, liquid, semi-molten). The lithosphere is a distinct layer due to its rigid nature, which allows it to be broken into plates that move on the weaker asthenosphere.