It only seems a dumb question until you know the history of the creation of the instrument - like that old saying - "Rules are for the Guidance of The Wise and Blind Obedience by Fools".

There were such beasties built in the early days when the instrument was first being created after the button accordion had pioneered the way - I have never seen the real things though, only illustrations.

As Nick guessed, it was found to be virtually unplayable because the left hand was providing the motive power (actually the shoulder genereates the power, the arm and hand only transmit it) for the bellows. This was in the days before small portable power sources like dry cell batteries and electric fans, of course... If the left hand keyboard was any different size than the right hand, you would not be able to play it 'like a piano'... but there is no way that it is at all 'like a piano' - the piano keyboard is used for many instruments - harpsichords, and pipe organs for instance, they 'play nothing like a piano forte'!

Indeed the Piano accordion is not a 'chest steinway' but more of a chest wurlitizer' - a portable pipe organ, using reeds (with register switches) - pipe organs do have reed stops as well as flute stops. The piano is a percussive instrument, the accordion - all types, including concertinas, are wind instruments.