Consumers observe the taste and cost of food directly, but checking the ingredients for healthiness takes extra effort and time. Rational inattention then implies that eaters are unlikely to verify the health claims. Thus food suppliers are subject to moral hazard: consumers buy based on the healthiness they expect, not the actual ingredients the seller chooses, so the seller has an incentive to improve taste, reduce the production cost and cut price even when this makes the food less healthy.
The standard solutions to moral hazard from economic theory are verification, repeated interaction and vertical integration (selling the firm). In the context of food, safety standards and truth-in-advertising laws restrict the substances manufacturers may add and claims they can make. Regulators verify claims made and punish for illegal additives or false advertising. Also, if a food supplier is found to use unhealthy ingredients (or amounts of sugar, salt and fat), then some consumers may switch to alternate providers, which is a repeated game punishment for the original seller.
The weakness of both verification and repeated interaction is imperfect monitoring: small increases in unhealthy substances are difficult to detect, because tests are noisy and food composition varies naturally. The variation sometimes makes the amount of an ingredient exceed the healthy limit, so honest suppliers would also be punished with positive probability. Incentives are created by the difference in payoffs, so reducing the payoff of the honest decreases their motive to stay honest. The imperfect monitoring allows unscrupulous sellers to outcompete the providers of healthy food on taste and price, for example by using various tricks to circumvent the legal requirements on labelling (https://sanderheinsalu.com/ajaveeb/?p=728).
The remaining solution to the moral hazard problem is vertical integration of the buyer and the supplier, i.e. home cooking. Of course, the ingredients bought to be cooked at home are subject to similar moral hazard – unhealthy substances can be added at any stage of the production process. The risk could in principle be even larger than for processed foods and restaurant meals, but in practice, it seems that simple and unprocessed ingredients are more difficult to manipulate than prepared meals, which are a mixture of many components. Adding sugar, salt, fat or monosodium glutamate to flour, rice or dry beans without mentioning it on the nutrition label is easier to detect than the same (amounts of) additives in shrimp fried rice, bread or a burrito. Raw meats and fish do have extra salt and food colouring added, but usually less than for ready-to-eat meals.
Relative prices are another reason why there may be less manipulation of ingredients than processed foods. There is a per-unit cost of adding unhealthy substances, as well as a fixed cost due to the risk of lawsuits and fines, especially if the additives are not declared on the label. Unprocessed ingredients are less differentiated, so the price competition is more intense. The increase in the price that customers are willing to pay if an ingredient tastes better than the competitors’ may be small if price is the main dimension of competition. The slightly higher price may not justify the per-unit cost of the additives. In contrast, for processed foods the margin may respond greatly to taste, motivating manipulation of the ingredients.
The taste of the final dish is likely to respond less to manipulating one ingredient than to altering the composition of the entire food, both because the ingredient may be only a small part of the final dish and because the taste of a dish is largely determined by the seasoning and the cooking method. In this case, additives to ingredients do not improve taste that much, reducing the profitability of manipulating these.
Intense price competition motivates cost-cutting, including by substituting cheaper ingredients or using additives (e.g. preservatives) that reduce the manufacturing cost. However, if the additives cost more than they save on production cost (such as preservatives for dry goods that already keep indefinitely), then they are unprofitable to include.
Demand for cooking ingredients may also respond less to price and taste than for restaurant meals or delivered food (raw ingredients may even be an inferior good, but eating out is more like a luxury good). In this case, there is a range of fixed costs of unhealthy substances for which adding these to ingredients is unprofitable, but to processed foods profitable.