I saw this behaviour with a very traditional European-born Mother and father-in-law. It was quite the opposite of my somewhat impoverished "not making anything new until the leftovers are gone" upbringing.

To this day, I'm eager to finish off the leftovers. My wife will be like, "That's been in there a week!!", and I'll respond with, "It's okay; I don't want to waste food. Every time you throw out food you're showing you're ungrateful."