"why not keep track of how much food my guests actually ate, and try adjusting the amount of food at my next party to match?"

Because the amount of food that people eat is not an absolute value, but a function of how much is there. If you do that adjustment, and then continue to do that adjustment, you will end with a situation without any food. That is true both at parties and in any other situation, like meals served to people who otherwise will have nothing to eat, at least to a first approximation -- if the last situation is absolute, you will get people eating some food, but it will not be enough to live on.