Now I know how hard it could be. There are about 35 pieces assembled with glue, flanges, and screws for each note that is played on the piano. In order for the note to play, these pieces have to move at exactly the right moment, for exactly the right distance, and stop at exactly the right time. The sequence of the movements and the distance between the parts must be very precise. Different piano manufacturers perform different qualities of regulation on their new pianos before they are delivered.
Some pianos are woefully out of regulation when they arrive in the home.
All pianos will gradually go out of regulation as they are played year after year as the felt parts compress. No two pianos will feel exactly the same when played, but ideally all the notes on one piano should feel the same.