Many of these vegetable oils are actively marketed to food manufacturers as a way to boost nutritional profiles with "heart healthy" fats. Additionally many of these products need the looser consistency of the polyunsaturats for the machinery (the easier the product goes through the system the less the system needs to be reinforced) and most consumers have thoroughly adapted to the mouth feel afforded by these oils.
"You can demonstrate the purpose and limits of human digestion with a simple experiment: eat a steak with some whole corn kernels, and see what comes out the other end. It won’t be the steak."
-J.Stanton