and family members use their own payment method to pay for things in family sharing

They can't use their own credit or debit card - any purchases they do will be taken from their account's balance (if they have one), otherwise the organiser's card will be charged.

but WHY??? what is the purpose of this restriction!?

Family purchases and payments - Apple Support :

With Family Sharing, one adult in the family—the family organizer—agrees to pay for any iTunes, iBooks, and App Store purchases that other family members initiate while they're part of the family group.

Why it's like that you'd have to ask Apple, you already know that I'm just a fellow user

Cutipatooti wrote:

Can family members use their own individual payment methods to pay for things in family sharing

Yes. They need to buy iTunes gift cards with their own money, and redeem the cards to their own account. Then their purchases (except for gift items) will be charged to their store credit balance rather than the Organizer's credit card.

For details, see Family purchases and payments - Apple Support .