Why deal with a full piano when you can just buy a Nord Stage 3 for $4500? Which by the way will do a whole bunch more sounds than just piano, such as an organ, various synths, etc?
Because, for whatever reason, when you put a brush on a canvas, or clay on the wheels, or you hands on a big Steinway, some piece of you hums along with the medium and capturing that resonance is what art is.
It's all sort of idiosyncratic at the end of the day, and how the thing feels in your hands is a big component of the artistic/creative process. If it works for you, it works. If it doesn't, try something else.