The reason it seems like that is because this kbin instance (kbin.social) isn't federated right now because of heavy traffic. So you'll only see magazines that were created here. As soon as the restrictions are relaxed you'll see other instances in the default feed. Or you could sign up for a different kbin instance like fedia.io which are currently still federated.
Even so, the other instances that are working as intended don't quite have that r/all feel yet just because there aren't nearly the same number of users in all of the fediverse as there were on reddit. It'll take time for the content to catch up.
It's a bit more general, but I always really liked r/WeAreTheMusicMakers. Sub for people who were creating music to talk about methods, techniques, and just different parts of the lifestyle in general. Also people could share their music, which does have the chance of being low quality spam but if you contain it to like a weekly megathread It's manageable.