Because Apple charges 30%. And Apple can keep that if there is a refund.
In this case, Epic is returning money. They would ideally be net zero on this whole debacle. You mistakenly bought $10 worth of skins, Epic collected that $10. Then you get refunded $10, Epic returns that $10. In the App Store scenario. You mistakenly bought $10 worth of skins, Apple collected that $10, gave $7 to Epic. Then you get refunded $10, Apple returns that $10, and then collects that $10 from Epic. Epic loses $3 on the exchange.
And if this is just refunding the purchases, it's kind of a good deal for Epic. As they essentially got an interest free loan from their customers.