It is not necessarily that the food itself is worse for you from a restaurant, but the fact that you don’t know how they make it. As @JLeslie said, they often add fats to things that may seem healthy and you wouldn’t even know it.
@Dutchess_III You say that pizza made at home would be just as unhealthy as pizza from a restaurant, but that is not necessarily true. This goes for any food, really. When you make it at home, you control what ingredients are put into the food, how it is cooked, and so on and so forth. You know how it is made and what it contains, and you are in control of that. You can use organic ingredients, lower calorie or substitute ingredients, any multitude of things that would be different from restaurant-made food. Restaurants cater to taste, so they can and will add anything to make the food taste better. At home, if you are more health-conscious, you can make dishes differently to make them healthier. Food made at home could be just as unhealthy, but the difference is that the choice is up to you, not some chef at a restaurant.