If you’re speaking about instant after they eat – other comments answered it pretty well.
If you’re speaking some time after eating:
This is because modern diet is based on carbs, on sugars. These get digested very quickly creating a spike in blood sugar. Your body then starts releasing large amounts of insulin hormone to get rid of it and overdoes it leading to insulin crash, where your blood sugar levels drop to very low levels temporarily. With modern diet sugar is your source of energy your body is used to so you feel tired when that happens.
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