Why does Piper maintain that performance art is alone partially unique?

In Adrian Piper's essay, Performance and the Fetishism of Art Objects, he explains his philosophical idea of how all art objects have their own uniqueness to it. Piper mostly focuses on art objects because of its solidity, structure and identity. When Adrian Piper states that performance art is alone partially unique, he means that the Performing Arts is something that can be only seen a few times, unlike art objects, which could be seen all the time by viewers and as many times they like. You can only enjoy a performing art at the moment, unless it is video taped, then it still wouldn't be looking at the same art, the video tape would be another piece of art itself.