Why can’t I buy anything with my Apple ID card instead of a credit card

Any balance on your account should be automatically used to pay for any of your purchases or subscriptions before charging your credit card. The exceptions to this are if you are trying to gift something, in that case gift card credit balance on your account cannot be used.

When using your Apple account balance, it may still require your payment method be functional and verifiable and ask you make sure its valid and working, but should not charge it if you have enough credit to cover the purchase plus any taxes applied in your location.