I have known of families where money is so tight that even energy on the meter is accounted for. And cooking from scratch can cost more in gas/electric than sticking a few things from frozen into the freezer and still having enough on the meter for a shower. I admit these cases are not the majority.
For a lot of people it's not just a case of being lazy either, more a case of if that's how you were raised and you're fine then it seems perfectly acceptable.
For others I know, where both parents work full-time, then pick up kids from childminders and don't get home until 6, cooking from scratch can take up valuable time spent with their kids, so again, it's easier to chuck a few frozen things in the oven. I think there's a lot of 'time-poor' people these days, and what they and their kids are eating isn't as much of a concern as getting some free time.
I do agree with the point that cheaper foods mean people with a bit more cash can eat more, which is what often leads to unhealthy eating. And I do think there needs to be more education around that. I work in a secondary school and frequently hear kids saying 'this is healthy though miss, says it's got real fruit in it' as they sit and eat sweets/drink fizzy pop. There's a lot of work to do around educating children about healthy diets, even how to make foods such as pizza healthier, they need to be taught how to make healthy versions of the food they like. I agree with a pp who said that food lessons need to be improved, in our school they do a lot of healthy eating lessons, and a lot of cooking lessons, but I rarely see them learning to make healthy dishes.