Why can't Wd's McD's and arbys offer a choice of wheat or grain buns.. if subway can do it why cant they?

Subway bakes its bread on premises as needed every day. McDonald's, as with most restaurants, gets its buns delivered. If a Subway runs low on Italian Herbs and Cheese bread, they can always bake more with little loss due to waste. If McDonald's were to try to keep the necessary amounts of both types of bread on hand, they would lose a lot of their product to waste.