It is because oil in a barrel contains many different parts. Refining is mostly just distillation. Chemically changing the product is sometimes done but is much

It is because oil in a barrel contains many different parts. Refining is mostly just distillation. Chemically changing the product is sometimes done but is much more complicated and expensive. With just distillation, one will get everything from butane to kerosene to fuel oil [1]. The light parts (lower boiling point) are generally more valuable, with fuel oil as the gunk left over (also used for asphalt). This is also why "light" crudes are more valuable than "heavy" crudes. [1] https://www.eia.gov/todayinenergy/detail.php?id=6970 https://www.eia.gov/todayinenergy/detail.php?id=6970