Why plants use oils but not fats as fats provide more energy than oils?

Fats and oils are the same thing. Oils are often plant based fats, while animal based fats aren't typically referred to as oils. Oils and fats both provide 9 kilocalories of energy per gram. Animal based fats are typically comprised of saturated and monounsaturated fats, while plant based fats are mostly polyunsaturated fats with some saturated fats and some monounsaturated fats.

Saturated fats are solid at room temperature, while oils (unsaturated fats) are liquid at room temperature. Some plant based oils high in saturated fats are: cocoa butter, coconut oil, palm oil, and avocado oil,... among others. Some like cocoa butter (used to make chocolate) have more saturated fats (57-64%) than pork fat (lard) [35%]. Though beef fat (and butter), at 80-95%, most definitely have the highest percentage.