Chips go stale because of chemistry. Chips lose the majority of their moisture during the frying process, creating what the Washington Post refers to as a "crunchy network of starch molecules." Losing moisture is how the chips get so crunchy. But, these little molecules attract moisture. So, when chips (and these molecules) sit out and are exposed to the air (and, especially humidity), they attract so much moisture that the chips get soggy. Putting them in the oven pulls the moisture back out so they're back to being crispy.