I once watched a Japanese person's video blog where he compared the ease of buying a meal in Japan and my home country. He said in Japanese cities small restaurants or street vendors are more numerous, selling cheap, quick meals of many sorts. So, people can more easily and affordably skip making their own dinners. He felt like the same goes for packed, ready to eat meals from convenience stores (I guess these would be the commercial bento in Japanese). I reckon this is one reason why it's so common in manga that people can't cook anything at all, up to the point of boiling water becoming a challenge.