asked, "Why is tuning a piano even necessary?"
First of all, there has never been a piano made by any company, at any price, that does
not require a schedule of regular tunings. It is also a fact that a piano will go out of tune whether it is played or not.
Humidity and then temperature affect the piano's ability to "behave",
so to speak. The average temperature and humidty is 70° and 42% humidity is comfy for a piano. If you have
a NORMAL metabolism and are comfortable then the piano probably is, too.