Here's why:

* Pumice is an extrusive igneous rock. It's formed from volcanic eruptions, with a glassy texture and trapped gas bubbles.

* Metamorphic rocks form under heat and pressure. These conditions alter the existing minerals and texture of rocks.

* Pumice is too porous and fragile. The gas bubbles and its glassy composition make it too weak to withstand the intense pressures needed for metamorphism. It would likely be crushed or dissolved rather than transformed.

So, while pumice can be weathered or eroded, it doesn't metamorphose in the same way as other igneous rocks.