Why are best modern pianists judged by how well they can play classical pieces?

Full disclosure: I just love certain piano pieces and I love the instrument. I do like listening to great pianists of today playing classics. But why are modern pianists primarily judged by how they play others’ music? It feels like this is somewhat unique to the classic instruments and particularly the piano. Shouldn’t genius be based on how well someone can create something new? It would be strange if modern painters were judged based on how well they could copy a Van Gogh…

With all due respect, just curious! Thanks!