In the case of ensembles with a piano, I've heard it's because you can't retune the piano in the minutes before the performance. So it makes no sense for musici

In the case of ensembles with a piano, I've heard it's because you can't retune the piano in the minutes before the performance. So it makes no sense for musicians to tune to a fork (or using an electric tuner) and leave the piano out of tune, so they all take the key tone from the piano