The staff ate it later

It's related to the concept of Mottainai (もったいない, 勿体無い) in Japanese culture. Where any waste is considered bad, specially related to food.

https://en.wikipedia.org/wiki/Mottainai

I wonder if this is why they tend to have plastic food displays at restaurant

https://en.wikipedia.org/wiki/Food_model

I think the plastic food displays are due to high uncertainty avoidance, so patrons can see exactly what their meal looks like before ordering. Yes you could use real food but the hassle of periodically filling the display case with freshly cooked dishes would be silly.

We should have more picture menus where every single menu item has a actual picture of the food served, instead of the guest trying to imagine the food based on often deceptive and flowery text descriptions.