I believe part of that is that usually the bulk of the cost of food is the labor and not the ingredients unless you have something 'extra fancy' with ingridents. If it takes as much work to make a cup of pasta as it does to make a quart or gallon of pasta.

An extra serving would add more labor as the expensive part and not save anything. The way to avoid that would be to keep a fresh surplus in the kitchen which would get wasted anyway.

Essentially it would unfortunately amount to 'efficiency theater' without very large assumption changes - sodas for instance have free refills because it is already a massive profit margin due to the sticky price-point where everyone expects it to be a dollar something while the actual cost is measured in pennies. You would die of water intoxication before they would realize a loss from refills.