Okay, let's get technical
Pianos go out of tune, fact of life.
The more often the piano is tuned, the better it will stay in tune,
and the more stable the tuning will remain.
So, why don't people tune with reguarity and frequency?
Well, they do. Those who want their instrument to
sound good consistently, tune consistently.
Annual tuning is the minimim. Once an instrument is tuned to pitch
(A440Hz, standard concert pitch) and stabilises there, the piano needs
regular tuning simply to keep the piano from losing that pitch.
So, annual tuning to A440 keeps a piano from losing pitch. What about actually tuning it to play and stay in tune? That's where more regular tuning does its job. On pitch and in tune with itself, octave to octave, throughout it's full range - regular tuning twice a year will help keep the piano on pitch and in tune.
Don't believe it? Well, if you doubt this prescribed tuning regimen, ask yourself why music schools and concert halls, recording studios and churches tune regularly and often even more frequently than every six months? Sometimes monthly tuning, sometimes daily! and usually before every single performance. It's not because they have excess tuning budgets, but because tuning changes with changing room environments (temperature and humidity) and hard playing, and tuning the piano is a necessity in order to ... simply have the piano be in tune and sounding its best.
