Disgusting?
Some people are showing their age here. Ironically (though not surprisingly) it isn't the thread-starter.
I can't understand how anybody could do that. Never mind why, there are many things people do that I don't understand, but it's the practicality of the thing that gets me. You'd have to walk with your knees and thighs slightly foward, in order for gravity not to take hold and pull the pants all the way down your legs.
Mick