This is gonna be the thread where we talk about stuff

That's largely due to dehydration. A similar experiment was conducted with a patty constructed out of fresh ground beef. It appears that the shape of the patty is more significant than its quality. Any patty with a similarly high surface area will dehydrate before decomposition can set in. Meanwhile, a fresh happy meal in a sealed plastic bag will mold quite readily. Also helpful are making the burger well done. Not to say that it's good for you, but it will decompose.

Citation: Serious Eats