I was never a huge salad fan. But for most of my life I’ve had fairly close control over what I eat; in the last year or so I have needed to buy more meals, and suddenly I understood a number of food-preoccupations that seemed vaguely comic to me before. This is not to say that I have ordered a salad in the last week or two, but I believe I can explain the obsession with salad.
First-order observation: when you make nutritional choices, you’re not just optimizing for one variable (total energy intake), you’re optimizing over several variables: calories, macronutrients, micronutrients, cost, time expenditure, and many other small details that are actually an important part of a healthy diet. Optimizing over many variables makes optimization problems much more complicated, especially in a problem with side-constraints!
Second-order observation: weight is mostly a function of hereditary traits + lifestyle, but which hereditary traits explain variance differs from person to person. The two major contributors are probably metabolism and conscientiousness; some have fitness-genes from one side, some from the other, some from both. The ability of highly-conscientious people to remain fit is contingent on their (a) having options and (b) knowing the consequences of the options. If you teach them that eating nutella from the jar and playing Zelda makes you thin, they’ll do that and won’t get thin.
Relevance for salads: salads are a relatively good way for anyone to hit their micronutrient goals while optimizing for a calorie deficit, and in many cases they are good way to hit macro goals as well (especially if you are a woman with relative low energy and protein needs to begin with, or if you have a lot of control over the content of the salad and can put a lot of beans/chicken/egg in it). But even more importantly, they are one of the few options at restaurants where you have a fair amount of control over/confidence in the ingredients.
The equilibrium for mass-market restaurants in a highly-diverse society is to serve highly-prepared meals, and in particular to serve meals which have generous amounts of sugar and oil added to the basic ingredients. Eating one meal which has added sugar makes diet-optimization a much harder problem for anyone. For conscientious people in particular, eating a meal where you have no idea how much sugar and fat were used to adulterate the food is a problem.
Many people who are trying to cut a few pounds would like to order just a grilled chicken breast, or something like that. Typically, not an option: if you order “the chicken” or “the salmon” it comes drenched in some mystery sauce swimming with lard and syrup. Trying to cut a deal where you pay the restaurant a few bucks for water while you socialize with your friends doesn’t work either. Thus, the salad.