The service providers are the ones who dictate the costs. They provide the infrastructure. The costs for these kind of transactions are much much lower because of economy of scale they handle millions of transactions per day across all their clients. Because they handle so many transactions they can charge a small percentage fee. The loss they make on small transactions they will make up with bigger transactions.

While Steam uses a normal bank transactions to pay developers, because many of them are in the hundreds of thousands and some are in the millions of dollars so you don’t want to have a third party handling those that asks a percentage fee. You’d rather just pay the fixed fee the bank charges per transaction. Since it is cheaper for those large transactions. That fee can be $10-$20 especially on international transactions. That’s why Steam waits till that money is above a $100. And using a third party to handle those small transactions wouldn’t be worth the hassle. The percentage fee would be high anyway because of the low volume.