Even with regular exercise and a stable diet is there a reason why somebody fat is not broken down?

When you gain weight your body fills your fat cells with excess energy in the form of lipids, if your body runs out of places to put lipids your fat cells split to make more room. When you lose weight, your fat cells shrink but they do not go away. Short answer, you have more fat cells than you originally had. The good news is if you maintain regular excercise and diet when the extra fat cells die they won't be replaced, the bad news is this can take years.