Here's why "food waste" is more frowned upon than monetary waste.
The top 1% might possess 99% of the wealth. But this wealth is mostly paper money. And there's almost equal food for everyone to consume. As in the rich aren't eating thousand times more than the poor, everyone eats the equal amount.
That's the justice of God. But when you waste food because it costs so less compared to the total wealth you have, you are destroying this balance. Spending your wealth to destroy things that the poor might have bought from their little wealth. That's a big sin.
You losing 10 million in gamble doesn't harm the society as much. It's just papers moving from a fool's pocket to someone else's pocket. But you spending that money to destroy 10 grains harms the society. Specially because it's food.
Which is why we have been taught a little bit of food waste destroys your wealth in large. That's God's way of stopping you from harming the community.