The person saving the last piece may remember who has not eaten yet.

Restraint at a table is frequently a form of accounting. Someone has been tracking who arrived late, who was upstairs with a child, who said they were not hungry an hour ago and probably was, and that tracking is why the plate is not empty. It looks like politeness or lack of appetite from outside. In fact it is a small piece of care performed silently, by someone who has been counting heads all evening.

There is usually one person doing this in every household and every gathering. Work out who it is, and make sure somebody has saved something for them.