Money. As explained to me by the director of the SFCC community band, it would cost literal thousands of dollars to have the entire orchestra coordinate to show up early before a concert to tune off stage. 100 some musicians, and the director, would all have to arrive, set up, and tune, to then go set up their instruments on stage, some of which would need to be tuned again. It’s much easier to set up once in place, tune in place, then perform. It’s also much cheaper.