Here's why:

* Erosion is a process that breaks down and transports rock and soil. It doesn't discriminate based on rock type.

* Metamorphic rocks, while harder than sedimentary rocks, are still susceptible to the forces of erosion. These forces include:

* Weathering: This is the initial breakdown of rock through processes like freezing/thawing, chemical reactions, and biological activity.

* Wind: Wind can carry abrasive particles that wear away rock surfaces.

* Water: Water can dissolve minerals in rocks, and its force can carry away rock fragments.

* Ice: Glaciers can carve out large sections of rock, and freeze-thaw cycles can break down rock.

* Gravity: Gravity causes landslides and rock falls, which break up rocks and transport them downslope.

The rate of erosion depends on several factors, including:

* The type of metamorphic rock: Some metamorphic rocks, like quartzite, are very resistant to erosion. Others, like slate, are more easily eroded.

* The climate: Wet, cold climates with frequent freeze-thaw cycles are more erosive than dry, warm climates.

* The topography: Steep slopes are more susceptible to erosion than gentle slopes.

* Human activities: Activities like deforestation and mining can accelerate erosion rates.

While metamorphic rocks are generally harder and more resistant to erosion than sedimentary rocks, they are still vulnerable to the forces of nature over time. This is why we see ancient metamorphic mountains slowly wearing down and transforming into sediment, which can eventually form new sedimentary rocks.