Why don't McDonald's fries decompose?
This controlled experiment of burger decomposition explains in detail why fast food burgers do not decompose easily. The same can be applied to fries, which are smaller and come dehydrated from the frying.

The main take-aways from this experiment are:

1: Dehydration is the main reason why fast food fries/burgers do not decompose easily. Placing the burgers in a ziplock bag, preventing dehydration, causes the burgers to decompose.

2: Since an unsalted, home-made burger did not decompose, preservatives, chemicals, saturated fats, and other components are unlikely to be the case of the fast food burgers not rotting.