"Oil" used to mean olive oil and nothing else. Its usage gradually expanded to include other vegetable fats, liquid animal fats such as whale oil, and finally

"Oil" used to mean olive oil and nothing else. Its usage gradually expanded to include other vegetable fats, liquid animal fats such as whale oil, and finally mineral oil when it was discovered. People back then weren't particularly interested in the chemical compositions of the different oils, only that they felt similar and served similar purposes. Meanwhile, animal fats that are solid at room temperature, such as butter and lard, were never really called "oil" despite being much closer to other edible fats than anything we dug up from a desert.