Why do some people eat SO much food that it does them serious physiological harm?

I have no idea what you're talking about....;-D....

What? What do you mean? I can't eat this?

To answer your question somewhat seriously and without getting into the very complicated details about hunger and satiety feedback mechanisms, the short answer is throughout most of human history we did NOT have access to the vast quantities of calorie-rich comparatively tasty foods we do today.

Hence, we are strongly genetically oriented towards eating as much of those foods as we can get (because, in the past, such access came about rarely).

Combine easy access to foods we are strongly motivated to eat with a major reduction in how much physical effort we need to exert to obtain them, stir in the modulating effects of constant low-level stress (which tends to cause the body to convert excess calories to fat storage), and you have the modern obesity epidemic.