because a prepayment system is trivial to implement and doesn't require a NIP. look at strike; they've built an entire business around allowing you to buy a bit of bitcoin, they measure the risk of default (ACH fucking sucks) and if you are not an asshole and revert they slowly increase your limit the world works like this, it's just natural; you don't treat your siblings with the same level of risk as you do a stranger. We are inference machines and we are evaluating/computing risk at every pivotal point. A service provider is in a great position to measure/price risk. But again, a prepayment system does not require a NIP, you just pay them and then consume the credit.