Pianos are acoustic instruments, which means they produce sound through the vibration of strings. The sound of a piano is naturally amplified by the instrument's soundboard, which is a large, flat piece of wood that is attached to the back of the piano. Guitar amps are designed to amplify the sound of electric guitars, which produce sound through the vibration of strings that are picked up by magnetic pickups. The sound of an electric guitar is typically much weaker than the sound of a piano, so it needs to be amplified in order to be heard.
Because pianos are acoustic instruments, they do not require any external amplification. In fact, using a guitar amp with a piano can actually damage the piano's soundboard. This is because guitar amps are designed to produce a lot of low-frequency sound, which can cause the piano's soundboard to vibrate excessively and become damaged.
If you want to amplify the sound of a piano, there are a few other options that are available. You can use a microphone to pick up the sound of the piano and then amplify it through a PA system. You can also use a piano preamp to boost the signal from the piano's pickups before sending it to a PA system.