And what happens when you've spent your allocated money on your allocated food and you have a child or family member with an allergy, so they can't eat half of it. And you can't buy more, because you don't have any actual money, just the card. Best case scenario, everyone ends up with a skewed rather than balanced diet.
LOL at the idea of getting a diagnosis for an allergy just so you can get an exception on the card, BTW. They can't even sort out their disability criteria properly so good luck with that one.
Or let's say you have mental health problems (10% of adults suffer from depression) and on a down, struggling day it's all you can manage to open the freezer and chuck something in the microwave/oven for the kids. Which is fine, until it goes on for three days and all the food in the fridge goes out of date meaning your family can't eat until the next allocated food date comes in. I'm sure that makes you feel great and helps you see the bright side of life.
Plus all the excellent points made by others.