Okay then. So, if bitcoin is a commodity, why do you think you can't really enter a contract "between two parties to buy or sell a specified asset (bitcoin) of standardized quantity and quality for a price agreed upon today with delivery and payment occurring at a specified future date, the delivery date"?
This futures contract (the definition of which is given above) is just one among many other possible types of paper derivatives
I never said this cannot be done. But because bitcoin is so easy to deliver, there is no practical sense in doing that. There is always a risk of non-delivery if delivery is delayed to the delivery date as is the case with physical commodities, so it makes more sense to buy bitcoin at spot price and withdraw from the exchange to your local wallet, and only deposit to the exchange when you want to sell. No point in storing bitcoins at the exchange while you're waiting to profit from the price appreciation, unless of course you're a day trader.
Physical commodities cannot be shuffled back and forth as easily as bitcoin, that's why future contracts in physical commodities are popular. It's a side effect that most people don't take delivery and just trade paper.