The cost of food is, broadly speaking, related to two things - quality and production.
Fatty foods are cheap because fat is cheap. Processed foods are cheap because they have cheap ingredients - cheaper processed meats and sausages are made from parts of meat which no-one would eat if they saw them before they are processed. Lean protein is expensive because it is expensive to produce - the animals have to be fed differently, and this is more expensive. Farming, if done conscientiously, is expensive.
Then there is the cost of getting the food to the consumer. Fruit, for example, is mostly hand-picked, mainly by people who aren't very well paid anyway. Hand-picking is time - and therefore money - consuming. We could have cheaper fruit if the pickers/graders weren't paid the minimum wage, but that's certainly not what I want to happen (although we know that it does happen, and there are many cases of seasonal agricultural workers being exploited in this way).
Many vegetables are more robust than fruit, and can be machine-harvested, which makes them cheaper.
Nor do I want to see milk sold at ridiculously cheap prices when I know that this is done at the expense of the dairy farmer who is being paid less for the milk than it is costing him to produce. And there is nothing he can do about it because if he doesn't accept the prices the supermarkets are offereing there is no-one else to sell to.
Supermarkets are interested in selling things. That's it. Cheap deals, bargains, etc., are not for your benefit - they are there to get you into the shop to sell you more things. Any concern they appear to show for our health and wellbeing either forced on them by legislation, or is window-dressing.
Governments are concerned with healthy eating because unhealthy eating means increased health-care costs. But in a free-market economy, they can't force shops to lower prices.
