Spices are expensive, salt is cheap. To make something taste good, you need to add a lot of flavor (spices). A cheap alternative is to oversalt stuff and add minimal spices. Salt makes the brain interpret other flavors stronger.
Glutamate (Umami) also has the same effect on the brain, so if needing a low sodium diet, you can add glutamate rich items instead of salt. But these are protein-based and much more expensive on a large scale than salt is as well.
Frozen dinners have high sodium content because sodium acts as a preservative and helps to extend the shelf life of the food. Without the added sodium, the frozen dinners would spoil faster and have a shorter shelf life. Additionally, the high sodium content also adds flavor to the meals, which is important because frozen dinners can be bland due to the freezing process.
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