That's exactly the problem. A good majority of fat people have never been skinny in their entire lives, or at least not past the age of 4. So they have a very warped view of what healthy looks and feels like. They don't think it's abnormal to constantly be in pain all day long, or that it's not normal to exert massive amounts of force to move your body around. It's no surprise they're such advocates of the HAES philosophy, they literally have no frame of reference!