Things that are made from organic material age and decay, especially when they stop being alive. A piece of home-baked bread, say, left on your kitchen counter, will get moldy relatively fast. Lord knows what some ground beef would smell like after a week. But the artist Sally Davies has been photographing one McDonald's hamburger and fries every day for 137 days. They look basically exactly the same.
Here is Day 1:
Here is Day 94:
Here is Day 137, which actually looks like things have improved in the 43 days since the picture above:
Far be it from me to tell you what to eat, but I am pretty sure your stomach can't break that down any better than the mold and microbes in the air. Check out the whole progression here.