Because that's not a "digital good" and you're not buying something from your bank. It's the same reason you don't pay Apple 30% to buy something with Best Buy's app or Target's app. The rules are not arbitrary. They're set up so that if an app is using Apple's store and infrastructure, it has to pay Apple for it and it can't circumvent that infrastructure. The alternative is a clusterfuck of payment systems and transactions with Apple as the middleman with no way to ensure any kind of experience for the customer.