I think the biggest factor is time, many families need and desire convenience. Another side to it is portion control; eating unnecessarily large meals bumps the cost.

I also find it strange that junk is cheaper... we earn very, very minimum wage (I'm at home, DP is an apprentice butcher) and we wouldn't dream of buying anything in a packet, it is just not worth the cost. When we need to watch our budget, first thing to go for us is meat and cheese in favour of bulking up with more veggies. We also grow veggies which has been a great investment (we have survived 'rego week' on $27 because of that gold mine lol), maybe I need to set up a charity that helps low income families create their own sustainable veggie patches... that seems a good idea to me! But we do have a lot of time, I can watch a pot making stock all day and don't have the stress of rushing around after a day of work to throw something together. I think time really is a big part of the picture.