Because some people prefer Pay-Per-Use rather than yet another Subscription. I am one of them; even my cellphone plan is Pay-Per-Use which can both be cheaper or more than a subscription, but in both cases I am in the driver's seat.
But adding credit to a wallet is not pay per use. Utilities like electricity are pay per use. I also prefer that, but I don't want to randomly guess how much money I need to put in a wallet for a service
