The problem is a large portion of the food in restaurants is not prepared in the restaurant. If you order a burger meal, yes the meat is cooked on the grill at the restaurant, but the buns, and any sauce were likely made somewhere else. They might purchase the meat pre-seasoned from the food distributor. And the french fries likely even have a seasoning pre-applied to them before they are delivered to the restaurant.

There are some places that are much better about this than others. I remember in Europe several of the restaurants had _books_ containing all the potential ingredients and cross-allergens for each dish. I distinctly remember Wagamama's in London pulling one out and double-checking it due to my spouse's eggplant allergy. Sadly it removed most of the menu that we were excited to eat, but it was damn impressive.