system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can a piano go years without being tuned, but you need to tune a guitar every time you use it?
gpt
Piano strings are strung on a cast iron frame.  Guitar strings are strung on a piece of wood (usually with some partial steel reinforcement in the neck). Environmental variations (temperature and humidity) will cause much greater variation in the wood in the guitar than the cast iron of the piano.

In addition, guitar strings are under about 18 lbs of tension, whereas piano strings are under about 180 lbs of tension.  Slight variations in that tension cause a much greater effect when the tension is less.