This is unsurprising, if it was kept in a dark, dry, climate controlled environment like a closet in an air conditioned home inside of a shoebox where there will be no airflow. Leave lots of things in such a place and entropic decay will be minimal.

That meat has been cured. That's why it looks so good. People have forgotten how to cure meat for long term storage since was have refrigerators.

legend has it, if you learn the secrets of the burger you can become immortal

A hamburger in your closet. Always someone crazier.

Maybe you could put one in a blender with water, turn it into liquid run it through a cheese cloth and use the filtrate as a fungicide?

It must be really toxic if it doesn't decompose considering how much sugar is in one of those things.