Just imagine if you had done meal plans and budgeted for things, to find out that when you want to make that macaroni Cheese for dinner, the cheese is all gone, or you go to make the packed lunches and there's only 4 slices of bread left... or you buy a pack of 8 mars bars, with the intention that each person has 1 each, and you go get yours, and they've all gone, and it turns out that Sam ate 4, and you have the others kids moaning at you that they didn't get theirs*
This is how it used to be in my house. Kids would help themselves to contents of the fridge on shopping day. We were on a VERY tight budget when they were small - single mum of five, two at home, no childcare, no job... it was desperate. I'd buy in enough (I thought) to get us through and then go to the fridge to find they'd eaten all the yoghurt, all the cold meat, and most of the cheese in one day that was meant to last a week.
They had access to fruit and bread to fill up on, but they'd rather have the 'high value' food. I tried to get it over to them that we didn't have the money to replace the food, but they'd rather have one day of stuffing themselves with all the nice stuff, than eke it out over the week.
None of them have eating orders now they are adult, by the way. It was purely competitive eating, so that nobody else could have the stuff they wanted.