When I was a child Mums didn't go out to work, they were housewives who had plenty of time to shop and cook sensibly, whilst managing their "housekeeping" budget and most were happy to do so (if not, they just had a job rather than a family). Mums nowadays are expected and encouraged to have full time jobs, sort out the needs of their children and still do most of the housework etc. They can however afford to buy ready meals rather than cooking so it easily becomes a lifestyle choice (like using a washing machine and tumble dryer rather than hand washing and hanging washing outside to dry n- and ironing - what's that?).
If one parent stayed at home until the children were at least in full time education and then took a part time job during school hours like our parents did (and indeed I was lucky enough to do) I think the stay at home parents would have time to learn to cook cheap nutritious meals etc. There is enough pressure on young mums who are doing their best to juggle their busy lives without being criticised for choosing the easier options foodwise. (For clarity, my grandchildren are fit and healthy and not overweight. My DD is a have it all ways mum but she is also a brilliant cook but just don't know where she gets the energy from (in fact, I secretly worry that one day she will just burn out like the city traders do and be forced to slow down!)