Why can’t my teen use his own payment method to purchase apps or in app purchases?

Why can’t my teen use his own payment method to purchase apps or in app purchases?

We have a family share plan that we still have ask for permission set up for me teen son. He has his own bank card and Apple Cash account and would be like to use his own money for in app purchase or apps that cost. Is there a way to allow him to select his own payment method while still being part of the family. I would like to see a feature that allows multiple payment methods for the family. I think this also is the same for my husband. We have separate banking accounts so having our apple one payment from my account is fine but we would like to manage and maintain our own purchases. Please help