Because of evidence. I have knowledge about watches, how they are built, how they are manufactured, characteristics that show evidence of being built. On the other hand, I have no evidence of any natural process that would result in a watch forming.
As I said, the watch should not be needed. If you are arguing from intelligent design, then everything is designed, and absolutely anything should be proof of design in your analogy. You should be able to pick up a rock, show it to me, and say, "You see, this rock is obviously designed."
Furthermore, I never said the watch is obviously designed and the forest obviously is not. You did. In your analogy.