Good question but have you ever thought where to send that? It takes time and more fees if rewards would be sent through fiat. And also it could be dangerous. But if we use digital wallet, it will take a short time to transfer and you can plan on what you are about to do with that, if you were to invest some part of it or withdraw. Also, if they sent you with the current price, you won't be able to hold it for you to gain more profit.