Why do i need payment information to download Facebook

Because you must have a payment method in your account to download anything, at least initially. It is used to verify your location and identity. If you have no subscriptions and no apps with in-app purchases you may be about to remove your payment method, but if there is one already on the account it must be active and valid. Here is more guidance→Change, add, or remove Apple ID payment methods - Apple Support

This is one method used to validate your identity.