Fossil fuels are complex materials and the refining process breaks crude up into several different compounds, these compounds have different uses with the base

Fossil fuels are complex materials and the refining process breaks crude up into several different compounds, these compounds have different uses with the base materials for most plastics being an, essentially, useless side effect of fuel production. It's less a question of taking a barrel of oil and turning it into a hunk of plastic or a can of fuel. Both are produced in the most efficient processes.