In my experience it's usually because human sized actual people, like you or me, can go an hour or more without needing to stuff thousands of calories into our faces. Obese useless hogs on the other hand, need to feel uncomfortably full at all times or they will literally die (or go into starvashun mode and gain 10lbs per hour). Also, certain foods permeate the air and can nauseate others, especially in close quarters. Humans grasp the concept of respect so they usually tend to be considerate of those around them, whereas obeasts operate on urges and if it doesn't directly affect them, then they don't care. On the rare occasion you do see a human eating on public transportation, it's something along the lines of a protein/meal bar or a piece of fruit, unlike eatbeasts that think nothing of gorging on a 3 course malodorous grease-fest to warm up for their third dinner when they arrive at their destination. I noticed this phenomenon quite often growing up in New York. Also, if obeasts can't eat it, steal it, or shit on it than it serves them no purpose. A bit off topic but it's something else I have observed in the wild.