Here's why:

* Lithosphere: This is the rigid, outermost layer of Earth, composed of the crust and the uppermost part of the mantle. It's about 100 km thick on average, but can be much thinner under the oceans and thicker under continents.

* Asthenosphere: This is a partially molten layer of the upper mantle that lies beneath the lithosphere. It's relatively weak and allows the tectonic plates to move.

So, while the lithosphere includes both the crust and the upper mantle, it's the combined thickness of these two layers that makes it the thickest part of the tectonic plates.