Why?
You can do the same with literally anything you like... so long as it is available in some standardised form. All they're doing is borrowing, selling and agreeing to return an equal amount of something.
I could borrow a bag of coal from you for example with an agreement that I return a bag of coal to you in future and I pay you a little bit in interest too.... I then sell the bag of coal and later, if the price of coal falls, I buy a bag of coal to return to you.
You have back a bag of coal you weren't otherwise doing anything with during the time I borrowed it and I made some money from coal dropping in price.