So the problem is often not "I have £50 to plan food for the rest of the month" but "I have an unexpected expense and now I have £2 I found behind the sofa and a bare cupboard/fridge to feed myself until I get some more money".
I also found that when I am poor I am much more reluctant to go out and buy in bulk. Even though it would at least give me food enough to last, there's something scary about blowing your entire budget in one go.
If you are poor it is also harder to get to places that sell things cheaply in bulk (because perhaps you don't have a car) and you are more likely to get things from a corner store as needed where there is a higher markup on prices.