Today in the Observer, there's an article suggesting that we should rename "fast food" as "cheap food", thus better identifying why it is that people buy it. The comments on the article, predictably, note that fast food is in fact not cheap, and that instead it is cheaper to cook the same meal at home.
The thing is, both of these things are true, indeed are true simultaneously. It is indeed true that fast food is not all that cheap, and that it would be far cheaper to buy all the ingredients and make the meal yourself... if you're starting from a position of knowing how to do all that and having all of the requisite equipment in your kitchen. From a standing start, though, it's much cheaper to spend £5 on a McDonalds, or indeed £20 on a McDonalds for four, than it is to buy everything you need to equip a kitchen to make yourself burgers and chips.
In the long run, of course, it's far better, and cheaper, to make the investment in a reasonable set of kitchen equipment, learn to cook, and then produce most of your own meals cheaply, quickly, and healthily.
Which, of course, brings us back to the Sam Vimes "Boots" Theory of Socio-economic Unfairness - having the time, knowledge, and equipment needed to cook all your own meals at home requires an investment that the rich can make easily while the poor cannot, which means that the rich person gets to eat home-cooked meals most days. Those meals are typically better quality and tastier, meaning that the rich get to enjoy their food more and live longer, healthier lives.
And the poor get to enjoy the privilege of paying more for this lower quality of life (measured over a lifetime).
Perhaps the worst part of all that comes when you ask the reasonable question: "how do we fix this"? Because, sadly, the answer is "we can't". The sad reality is that the rich will always have the ability to make strategic investments that allow them to (a) enjoy a higher quality of life and (b) pay less in the long run.
(In the case of cooking meals, we can at least make a few inroads - we could prioritize the preparation of food as schools, thus ensure everyone leaves at least knowing how to knock up a few cheap and healthy means, and we could insist that all houses that are built and/or properties for rent must have adequate facilities. That would at least reduce the inequality a little... but it does nothing any of the other examples.)
Ultimately, those inequalities are probably just inherent in a capitalist society.
Although... it's always worth bearing in mind that even if all we can do is nibble around the edges of the problem, it's still probably a good thing to do that - the result might not be much, but at least it is something.