Motor oil has to be refined as well. This is why the nice gents at the gas station always ask me what kind I want. “10/40 or 20/50?” Naturally, I have no idea what they’re talking about, except that they are different petroleum fractions, separated by refinement.
As to why gasoline refining leaves it so much cheaper than refined motor oil, I have only guess:
(1) Demand for gas is higher. Everybody needs both, but you don’t buy oil in the same quantities. So, the process for gasoline refinement have a cheaper unit cost (bigger facilities, cheaper additives, etc.)
(2) Maybe there is just more of the stuff that goes into gasoline than the stuff that goes into lube oil. Gasoline comes from the “middleweight” range of crude oil (septane, nonane, etc.); viscous oils come from the “heavyweight” end (dodecane?), where concievable there could be less around.
(3) There is some sinister cartel raising lube oil prices to what the market will bear. This isn’t happening nearly as much with gasoline, because we buy so much more we just couldn’t afford the cartel prices. This theory is pretty bogus, since in Europe they manage to afford gasoline prices several times what it costs in the U.S.