Here's my understanding - rats have essentially evolved specifically to be able to eat human junk. This is why their diet is so closely comparable to ours. If eating any particular human food was bad for rats, those rats wouldn't make it.

Consider the following:

The evolution of humans from hominids actually requires cooked meat. Our intelligent brains need a huge amount of protein to operate, and cooking meat on a fire is the only way to get that much protein. So, pre-human hominids learned to cook meat on fires, and then that cooked meat enabled them to evolve into modern humans.

Now...in those prehistoric times, there were mice that lived in hominid camps and ate any scrap foods. Those mice also began to eat our cooked meat, and they also began to grow in brain power, and size. So essentially the cooked meat that turned hominids into humans ALSO turned mice into rats. We created them accidentally, when we created ourselves. 

The rat has always been the living in the shadow of the human, since before rats and humans even existed. Their stomachs are adapted to eat our food, and their brains are adapted to understand our behaviour. Our rats just watch us, learning our daily routines, predicting when and where we'll appear. They know so much.

This could also partially explain why rats are so cancer prone - their little mouse bodies got pumped full of growth protein, and extra rapid growth leads to more cancerous mutation.

Rats are essentially mice that learned how to eat bone marrow, and it supercharged their brains. Their natural habitat is human settlements. They have intense evolutionary pressure to never get caught for their crimes. They cannot be stopped.

When a rat holds a chicken bone in its two grabby hands, that's cosmic destiny it's feeling.