because it its highly processed.
There are two points:
- most vegetable oils are extracted at high heat and with chemicals, destroying their nutritional value and creating some trans fats that are very bad for the body, which is why most vegetable oil you consume should be first cold pressed, and preferrably organic - olive oil for example or coconut...
"Today’s technological age is enjoyed by the fattest, laziest humans in the history of humanity." -Mark Sisson