Re: "Why does Apple make me add a card to download apps when a card is already connected".
We are all Apple users, like yourself in the community, so we have no access to check your account or details.
Wondering if the expiry-date on the card in use is still valid, or whether an unpaid purchase may be involved.
If a message says 'billing problem with previous purchase' or 'verification required' - Apple Support
or
You may want to check which payment methods Apple can accept in your country or region:
See: Payment methods that you can use with your Apple Account - Apple Support
and
If needed: How to: Add a payment method to your Apple Account - Apple Support
You may need to allow 24 hours for recent changes or updates to come into effect.
Else: Restarting your iPhone may help update / display / any recent changes, as that re-establishes connections.
______________________________
If you want more support / answers from the experts: You may want to reach out to the Billing Support Team:
There is a "Get support -start now" link (online chat / phone / email etc.) at the bottom of their webpage below:
Subscriptions and Billing - Official Apple Support