This is why your plastic ketchup bottle still smells like ketchup after washing it multiple times - at a microscopic level, plastics essentially look like uncooked ramen/instant noodles, basically a big tangle of polymer chains which act like a sponge.

Now, consider what happens when you wash that plastic with a cocktail of chemicals in your dishwasher...

Obviously very hard/dense plastics (melamine comes to mind) are less prone to this