Back in the day, oil wasn't as abundant and cheap as today. Just like sugar (or sweeters in general). The fact the family isn't familiar with deep fried dishes proves it as well. If oil was plentiful, they would already have an extensive cuisine of frying food.
But then again, I doubt the author really thought things that far. Exactly 0% of the details in this manga have a lot thought behind them. Might be intentional by the author or maybe that's the limit of the author's skills.