De Novo Lipogenesis is simply the conversion of other energy substrates (carbohydrates, and to a much lesser extent, protein) to fat. Carbohydrates can be converted and stored as bodyfat, but dietary fat cannot be converted to carbohydrates. Well, technically the glycerol backbone of fat can provide a little bit of glucose, but it's miniscule.
Ultimately, it's why calories matter most for weight loss, because even without dietary fat, the body can store fat (or maintain fat stores) from eating carbohydrates.