I think its because of the shape of their body. On a healthy man, his hips are typically slightly larger than his waist, so the waistband if their underwear kind of sits above the hips as it is supposed to. However, because fat slobs' waists are bigger than their hips, the waistband has nowhere to sit and just naturally falls down their body. I'm not sure if they are not able to feel this happening or if they just don't care because it happens all the time.