I think it's also about trade offs that are made. For example, TV dinners (from an article in another thread here) for us was not a staple, it was a luxury. They kept for a long time, but they were also high cost to calorie ratio. We got the cheapest cut of meat we can in bulk and frozen it.
While I'm not suggesting your method isn't more economically efficient, it also requires time and money investment. When I was "poor", my parents would often both work late and it wasn't uncommon to not see them until later in the night if at all before bed. Many nights my brother and I would make our own dinners. Dealing with large cuts of frozen meat takes planning and skill that microwaveable dinners do not.
Also equipment: one thing working with our local mural aid group has really underscores is how many people have, say, a microwave or hot plate but not a working stove or enough capacity to store bulk food purchases. That’s definitely not true of everyone, of course, but it’s a real barrier for some people- especially, say, a newly-single mother who can’t feed an infant rice and beans.
