I think there are several factors in play. Loss of Cookery lessons may have some bearing, but since I left school I have never cooked junket or soused herrings! I learnt from my Mum who was not a good plain cook, but would spend hours making 'fancy food'.
Another factor is that a working mother or father, after putting in an eight hour day, is not going to be able to make a meal from scratch and eat before 8pm. Sometimes you can prepare a meal the night before, but who wants to peel potatoes at 10.30 at night!
Finally, some children will only eat rubbish unless you want a fight, the culture of giving in to children's every whim is having an effect on their,health.
You can get it right, but it takes a lot of time and effort on behalf of the whole family.
Ps: I did notice on Newsnight that a child getting his school dinner had two potatoes, plain pasta and a Yorkshire pudding on his plate, no meat or veg!!!