Petroleum cokes sells for a small fraction of what the transportation fuels sell for.
I used to work as a chief engineer for a company that designed and built cokers for refineries. The entire operation of a coker is to produce as much lighter grade fuel, like gasoline or diesel as possible.
The coke is the leftover waste. It has the smallest value of the separated products from that unit. They are designed to minimize the coke produced and maximize the lighter units produced.
The coke is the leftovers, as is asphalt. But when you get your heavier grades of crude, you still get a lot of coke, regardless of how much your cracking capacity is (I suspect there is a diminishing returns thing there).