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.

Some American restaurants have real food displays, too. With a chilled display case and limited airflow (and choosing only meals that keep well - avoiding exhibition of garnishes or salads that wilt in hours), you can put the same dessert on display for days.

At the end, of course, you have to throw it away - it might not be safe for staff to eat by the point it's visibly decomposing from 3 feet away. I find that just knowing the food in the case is destined for the garbage to rankle, especially when I'm simultaneously looking at menu prices and wondering why the meal costs so much; it's interesting to learn that the Japanese make those meal displays out of plastic/wax for the same reason.