The answer to this question became quite obvious to me after taking my daughter to a very popular restaurant chain last weekend.
Each entree contained at least 900 calories (with most salads exceeding 800 calories per portion).
Knowing that we burn about 15-20 calories per minute doing the most strenuous types of exercise, I asked myself how long would it take me to burn off those 1000 calories. 1000 divided by 20 comes to 50 minutes of running (!) for every lunch entree on the menu in front of me. And that's just not to gain weight! Imagine how much more exercise it would take to actually lose weight! For some reason, i lost my appetite at that point and ordered 1/2 a serving (with 1/2 the calories).
So, the simple point is that our modern servings come in such huge sizes that it is unrealistic to hope to burn that energy off by exercising. Please, don't misunderstand me here - exrecise is very important to for health, but obesity is primarily caused by excessive calorie intake (and treated by reducing that intake).