Here's why:

* Mantle: The mantle is the thickest layer of the Earth, located beneath the crust. It's made up of hot, dense rock.

* Asthenosphere: This is a partially molten layer within the upper mantle. The extreme pressure and heat cause rocks to behave like a very viscous fluid, allowing for movement and melting.

While magma can also form in the lower crust, it's primarily generated within the asthenosphere due to the higher temperatures and pressures found there.