Because that is Apple's policy. If your account is telling you to put a payment method in your account, then you will need one in order to get free apps, redownload apps, and update apps.
Your bank card will only be charged if you actually buy something.
An easy way to have a payment method if you don't have a bank card is to buy and redeem an iTunes/App Store gift card. Then, you will have the credit balance from the redeemed gift card that will serve as your payment method.
M1 Mac mini, macOS 15.3.1; Watch, series 10 GPS-only, watchOS 11.3.1; iPhone 16, iOS 18.3.1; TV 4th Gen, tvOS 18.3; HomePods, audioOS 18.3 and Airport Express w/ Airplay 2