There is a valid scientific reason for this.

Your body requires a certain amount of carbs protein and fat and you are subconsciously drawn to foods that have the proper proportions of these three items.

Potatoes do not inherently have much fat or protein so we add things like sour cream and butter because they are high in fat and then we eat them with steak for protein.

French fries on the other hand is bathed in pure Oil making them really high in fat fulfilling two of our bodies basic needs fat and carbs, we then eat them with burgers that are high in protein. The ketchup has little to do with this specifically however there is a valid reason ketchup is full of citrus acids an acid commonly found in fresh fruits deceiving your brain into thinking you’re eating a healthy delicious fresh fruit instead of a starch filled oil drenched root.