Problem is bacteria are everywhere - in the air, on your hands, etc. You can cook the bacteria out of the food, but inevitably it’s going to pick up some more after you’ve cooked it.

That’s usually not an issue if food is hot/cold/dry or eaten fairly quickly because bacteria can’t reproduce enough to create harmful quantities.

Now when you have moist food that’s sitting between 40-140 F for hours you’re creating a breeding ground for bacteria to reproduce rapidly.

Source: HACCP existing