Because when you finish the ritual of giving food (from Buddhist scriptures), the food that ghosts eat is different from the food you see.

You can watch the Mantra of Giving Food and Manna-Burning Mouth in my space! 》

Etiquette comes from

Dalagni Classics of Buddha's Theory of Saving Hungry Ghosts and Pulling out the Flame Mouth.

The Buddha said that manna passed through Dalagni.

You might as well read the Buddhist scriptures on it yourself.

I clearly know how the Buddha explains "giving". "Give food", what is "food" for ghosts to eat?