This was from quite a while ago, back in the 80s. I remember a textbook that when we got to the section about nutrition, it only taught us about the food groups. So if you ate all your food groups at a meal, it was considered to be a healthy meal. Pizza and hamburgers were specifically mentioned as nutritious because they "included all the food groups". So they were considered to be a rich source of nutrition, whereas a bowl of spinach would be a poor one because it only contained one food group.
Also there was a big emphasis on getting enough servings of each food group every day, and no emphasis placed upon portion control.
For example it said you need 6-11 servings of carbs, without telling us how much a serving was.
So this in a child's mind could transfer to 6-11 full bowls of cereal, 6-11 sandwiches, 6-11 potatoes, etc. because there was no guideline stated in the book about how much a serving was. Some of the examples of foods in that food group showed full loaves of bread.
It also said that fats like butter and sweets should be eaten "sparingly" with absolutely no guidelines as to how much to eat.
Pizza was said to be healthy because it contained Pepperoni (the meat group 2-3 servings daily), cheese (the dairy group 2-3 servings daily), tomato sauce (I think it was around 5-6 servings of fruits/vegetables, but I can't remember exactly), and the crust (carbs, 6-11 servings).
By this logic hamburgers, tacos, and nachos would also be the epitome of healthy food.
Spinach alone wouldn't be healthy because it's only one food group.
With textbooks like that to teach us what was healthy and what wasn't healthy, it sheds some light on why so many people my age are fat, in addition to other contributing factors.