Why can’t I share purchases but also pay for my own media in family sharing?

2wister,

You can pay for your own purchases. Purchase gift cards at a supermarket or other retailer, and use them to maintain a sufficient balance on your Apple ID. See details in here about how it works:

Family purchases and payments - Apple Support

Only the Family Sharing handler (the Apple ID of whom created the FS) can pay for the charges. You may purchase and download them from your device, but the charges would go to their account only.