What you say about controllig diet is true. But you also have to take the metabolism rate of the individual into consideration.
I have a friend who eats twice as much as me (he gets hungry every 4 hrs or so) but is shorter AND quite thinner than me.
I never used to care about my health and one time I hit 196 lbs/27% body fat for 5'8" and my doc said I'm getting into the obese category. That was the wake up call for me. I exercised a lot (5 days a week + 1 hr swimming on Sat) to bring it to 175 lbs/18% body fat. I then let go (no exercise) and I now hover around 175lbs to 180lbs for 25% bodyfat.
The reason I'm around 175-180lbs is because I control my diet very well. But there is no way I could go below 175 with just diet control (heck I couldn't go below that even with exercise). I definitely can't improve my diet further. I already eat only a 6" sub for lunch, cook at home for dinner and just have a slim fast optima for breakfast. And I'm vegetarian.
Long story short, for some people even just dieting won't cut it and adding exercise will just improve that a little bit. May be for some of the very obese people, their metabolism is very bad.
Not saying that's an execuse for all obese people. I have certainly seen many obese people stuffing themselves with shitty food. Just saying don't generalize it and say just exercise and dieting would be enough.