That really makes absolutely no sense. Derivatives whole purpose is to allow people to hedge their exposure to the underlying, in this case oil prices. Many parties active on these markets have such exposure but are unable or unwilling to take delivery as part of that hedging contract..

The people who take delivery hate that. They want someone else to pay for the oil that isn't in their possession yet. There is a lot of money worth of oil sitting in transport and the people who trade oil are the only ones who want that much money sitting around. (often people play the same market, but they like the separation anyway)

Why?