well thats because of what goes into playing a grand piano for time on end.
no painter requires precision, speed, quality of touch in every finger. they jsut need to know how to stroke. and a painter doesnt paint infront of an audience and recreate a painting without mistakes, a pianist needs to. infact if you put a painter in the same spot as a concert pianist. lets say this situation
there's a painter, and easel, and w/e colours he needs, and the painter is to re-create the mona lisa as his performance, like a pianist might play a piano concerto.
he wouldnt be able to. he'd make mistakes. it would take him a very long time to get the strokes right, the colours mixed correctly, etc.
a pianist can recreate these pieces fairly flawlessly because he practices for it.