As a former guitar collector, I had guitars that I played and guitars that stayed in their cases. If improved technique was the reason for improved sound, then I should have noticed the improvement in all guitars. Instead, I noticed it in the guitars that were played regularly.
I've heard about methods to improve a guitar's sound by leaving it in front of speakers, or using the suction cup attachment to a Conair massager, or other methods to make the guitar vibrate for an extended period. I don't know of a method that beats frequent playing.
“Instead, I noticed it in the guitars that were played regularly.”
Then you agree, you heard a distinct difference in the guitars you played vs the ones that stayed in their cases.