A major reason is time and money. The cheapest and fastest foods available are often high in saturated fats and sugars. If you are hungry and want to have a snack, It actually takes some time to prepare some healthy meal - for example a tasty salad- and more time to wash dishes and so on, while you can have a pack of chips or even a hamburger at a fast food in no time. Even when the healthy meals take no time at all, they can often be more expensive.
Another common problem is the size of the servings. When the servings at a restaurant are too large, people will often finish off or take away their meal, even if they were already satisfied, because the idea of wasting food is not easily accepted.

For those gourmets with a large budget, and who loved Disney’s last animated movie “ratatouille” a great -but expensive - diet plan would be to take the habit to go to “nouvelle cuisine” french restaurants -if you live in a big city or are lucky enough to have one nearby-. The food is really tasty and delicious but the portions are often so small that will leave most starving. Great to apply as a diet plan.