One aspect of the problem is this idea that seems to have crept in, that it's necessary to eat elaborately cooked meals all the time. Making a couple of cheese sandwiches actually takes a lot less time than hitting the nearest drive-through, as well as being both healthier and cheaper.

I'm not so sure about that.. I'd think that a dollar burger and a dollar chicken sandwich is more nutritionally balanced than a cheese sandwich, preservatives and fat be damned.

There's also the staples, rice and beans and whatnot, but that's boring as heck. Fast food also wins on tastiness.. and since we're not robots, that's vitally important.

Your second paragraph is the answer to the first. The issue isn't nutritional balance - a good multivitamin pill costs ten cents per day if that's what you're worried about. The issue is that fast food is designed to exploit security flaws in your motivation system. That's one of the reasons Americans in particular are suffering so badly from the obesity epidemic despite food being available in effectively unlimited quantity in every developed country. Doubtless given another few thousand years, evolution would patch some of the security flaws, but that doesn't do you any good right now. All you can do as an individual to protect yourself is stay away from fast food.

In theory you could cook once weekly, and freeze everything.

If optimized it could take only a few hours,and could be a shared family activity.

But it's still work, and being a working poor is exhausting so i understand them preferring to rest.

I live in a very small apartment and only have a very small freezer inside of my fridge. I suspect that many poor people may not have enough space for a big freezer, not enough money to get a decent efficient one and/or not enough money to pay for the electricity.