Perhaps this is the key?
From the business's perspective,...
The business gets to pay its servers less than minimum wage, and also motivates its servers to steer customers toward higher priced items and more food. And I've seen that phenomenon blatantly at work sometimes, like when a neighboring couple was clearly on a date.
why in the world is the expected amount of the tip tied to the price of the food???
I've never liked that, either. On the lower end, we have a minimum tip we leave for dinner, figuring that the server had to work as hard even if we ordered an inexpensive meal. On the higher end, it doesn't seem fair to the customer, even if it's easier to calculate. Though I don't see why a fixed amount per appetizer, drink, meal, etc. would be any harder, especially if the restaurant printed a suggested amount on the tab.
Speaking of calculations, I saw something interesting about how tip inflation supposedly progressed (and I remember when the expected tip for good service was 10%). To make it easier to calculate, the tip was supposed to be twice the tax on the meal, but as the food tax became larger in bigger cities, the expected tip became larger... No idea if that's apocryphal or not.