gingerbear
as an Italian I can partly answer your question: we eat a lot less convenience food, and the vast majority of mothers work there too (actually, probably more than they do here, I don't know many SAHM in Italy, but know lots in the UK), it is I think a question of habits.
My mum has always worked full time 8to13 and then 15to19, and she's always cooked lunch and dinner.
She did sometimes resort to buy ready cooked food, but in Italy there are plenty of places, rotisserie style, family-run, that cook food and sell it the same day. It's still as good as home made stuff.
They eat less fat, much less butter, (preferring olive oil), they drink less (or at elast they don't do binge drinking as the british do - no offence meant to anyone).
It's true that soemtimes it's the lack of time, but I tend to cook in bulk and home freeze stuff (especially for dd), and cooking a plate of pasta doesn't take much longer than heating up something in the oven, you can make the sauce while the pasta cooks. Also if you're really short of time, what about a salad and some tuna, or cheese, or poached eggs? There's plenty of healthy things poeple can cook without resorting to convenience food.
Sorry if I keep repeating myself, but it is a quetsion of habit, and culture, possibly.
One last thing, Italians too these days complain that children are getting fatter than they used to be, and as far as computer games and sport are concerned they're just as bad as in the UK.
But still I think you see far less fat people around and it must be down to the type of diet.