Why is granite considered a metamorphic rock?

It isn't. It is an intrusive igneous rock. It forms by the solidification of bodies of magma relatively deep below the surface. Granite if it undergoes metamorphism is changed to a number of things, Charnokite is one, Metagranite it another, it can also be recrystallized to Schist or Gneiss. read more

Granite is a magmatic rock and is never considered to be a metamorphic rock. It is formed as a DIRECT RESULT of cooling of magma below surface . read more

PARTIAL melt granitic type rocks are 'granitic gneiss' which is a metamorphic rock. Granite-type rocks (granitoids) are about the most common - or THE most common - rock type in continents. Some form from 'primary 'melts (M type) or from the melting of pre-existing igneous rocks (I type) and some from the melting (due to burial and tectnonic heat and pressure) of sediments (S type). read more

Once a metamorphic rock is melted it is no longer a metamorphic rock and is a magma, so these rocks are seen as a transitional between the two, but are not technically granite as they do not actually intrude into other rocks. read more