They do, and they are! But only really with concert grands prior to a concert. Not as far out as maybe a guitar or violin might be, but just a subtle tweaking.
Guitar and violin, for starters, have exposed tuners which get knocked easily - in and out of its case, for example. Pianos are different. The mass of a piano also means its tuning is more stable, but over time it will still go out of tune, humidity and temperature differences being the main culprits - which also affect guitars and violins more rapidly, and to a greater degree as well.
Stretching strings on guitars will affect tuning, and that's one thing we don't do on pianos!