I think that's just the thing- they fucking know exactly what to do.  They know it's as simple as moderating what/how they eat.  They fucking know that their current state of mass is their own doing, but if they can for one second maybe convince their family, friends, the public, and themselves otherwise, then they will because it's easier than doing what it would take to return to normalcy.  The delusion is so, so strong it warps time and space.