Sith Empire/Old republic is like the Kardashians...

Nihilus did not destroy planets he killed everything living on the planet, the planets were still there and whatever was built, he ate the life force of anything touched by the force. He was drawn to places with many force sensitives and places rich in the force but he could not blow up planets or destroy them like the death star and other super weapons.

Nihilus was powerful but a lot of people clearly do not understand how his hunger worked.