In extracurriculars, you have one thing you simply don't
have in the classroom-the ability to say "my way or the
highway".
As a music teacher, I have taught both co-curricular and
curricular classes at the same time, and I can attest that
it is MUCH easier to teach a group where every single
student wants to be there, and where you can say "if you
don't wish to be part of the group, don't let the door hit
you on your way out" than if some could care less about
being there and have nowhere else to go, even when there IS
a performance that all need to participate in as part of
the group looming in the future.
I'd also point out that while there are some "everyone gets
to participate" activities out there, most performance-
based programs have ways of weeding and localizing students
who are less capable even if they take all of them, from
handing that child the "Special" tambourine part in
elementary school choir to warming the bench or being
equipment manager in high school sports.
Apples and oranges don't have much in common beyond being
fruits, and extracurriculars and classes don't have much in
common beside both being an adult teaching a group of kids.