Food "goes bad" when a lot of air passes over it and microorganisms like bacteria and mold get onto it and start eating it.

When the factory puts food in jars, they seal the jars so that no air can get in. Then they "pasteurize" it - they heat it up hot enough to kill any microorganisms that might already be in the food.

So what you end up with is a jar of food with nothing alive in it and no way for anything to get in.