Here's why:

* Convergent boundaries are where tectonic plates collide.

* This collision can result in:

* Subduction: One plate dives beneath the other. The denser plate (usually oceanic) sinks into the mantle, where it is recycled.

* Mountain building: Plates collide and buckle, pushing up mountains.

* Volcanoes: Subduction can melt the mantle, leading to volcanic activity.

In all these cases, existing crust is destroyed or deformed, not created.

New crust is formed at divergent boundaries, where plates move apart and magma rises from the mantle to fill the gap.