That the tip is a percentage of the bill makes no sense whatsoever. Bringing you Filets de venaison frits sur leur sauce a l'echalote is no more work than bringing you a Sloppy Joe with fries.
The only explanation I can think of is that it is a way for restaurant owners to encourage waiters to encourage customers to buy expensive, high-margin food.
According to a food-aware friend of mine, the waiter who brings you Filets de venaison frits sur leur sauce a l'echalote should be able to explain to you exactly what ingredients went into the food, how it's made, and a lot of other intricacies about your meal.
That's what working on commission does. Tipping is just working on commission but with a bunch randomness and social rules as obtuse as victorian era manners.
The only explanation I can think of is that it is a way for restaurant owners to encourage waiters to encourage customers to buy expensive, high-margin food.