I’ve been asking for this since iOS 7 first introduced the family plan. Initially, they set up family plans so kids wouldn’t be allowed to purchase anything on their Apple devices without parental permission. It evolved to family sharing, where a family member could share their purchases.
What gets me is that I’m a grown adult who still can not make my own purchases while still being part of a family plan because my dad is the organizer as he should be, and therefore, only his credit card can be uploaded. After 21, if you choose to stay on a family plan, the organizer should not be informed of your purchases and should not be the only one who can upload their credit card for purchases.
The only way around this is to buy digital iTunes gift cards from Amazon. If you want to buy a subscription, you have to go through the company instead of through an in-app subscription.
It’s really stupid that I as an adult who wants to be on my parent's family plan can’t upload and use my own credit card.