The same reason you never walk across someone’s putting path on the green of a golf course. Etiquette. Generally, you wait until the composer turns to the audience after the last note of the piece is played. Then the composer will give acknowledgment to the soloist to stand up and take a bow. That’s the proper time to hoop and holler.
And I'm sure it was autocorrect that changed "conductor" to "composer" ;-)
But, yeah, etiquette. Just like you don't talk during the performance. Or maybe it was tradition since my parents taught me these things when I was a four year old attending orchestra concerts,
If you want some kind of practical reason, one is that it interrupts the flow of the performance or is otherwise disruptive. It also puts an emphasis on one person even though the entire ensemble contributes.
I think there was a time when something was considered a "performance" (or a concert) and that dictated expected behavior. Much of the rules were to allow other people to enjoy the performance. But now many things that might once have been considered concerts are now "experiences" and expected to include singing along, talking during the performance, dancing, consumption of various substances. Did the member of the audience purchase a ticket to see and hear something or were they expecting another kind of experience?