As a guest, I also know that my food order includes wait staff setting the table, perhaps sweeping the floor around the table and polishing the silverware, bringing several dishes and utensils to the table, keeping water glasses filled, clearing the dirty dishes and scraping the food off plates and cleaning the crumbs and spilled food from the table in front of me.
Why shouldn’t I tip more for the food than the wine? When you add that there is seemingly no more effort needed for an expensive bottle of wine than an inexpensive bottle why should I tip a certain constant percentage on the total bill for food and wine when sometimes the wine is over 50% of the bill?
Guests often think like this and it can be hard to disagree with their view.