How is it that we know so much about now-lost Greek sculpture?

Some Roman sculptures (that we still have) are copies of Greek originals.

The sculpture of ancient Greece from 800 to 300 BCE took early inspiration from Egyptian and Near Eastern monumental art, and over centuries evolved into a uniquely Greek vision of the art form. Greek artists would reach a peak of artistic excellence which captured the human form in a way never before seen and which was much copied. Greek sculptors were particularly concerned with proportion, poise, and the idealised perfection of the human body, and their figures in stone and bronze have become some of the most recognisable pieces of art ever produced by any civilization.